* Checking whether the allocation succeeded may not prevent exceeding the memory
  limit excessively.
* Even if the memory limit is not exceeded, the allocation can fail theoretically
  and the program would terminate abnormally.
  This however is true for all allocations in SciTECO (via glib).
* teco_memory_check() therefore now supports checking whether an allocation would
  exceed the memory limit which will be useful before very large or variable allocations
  in addition to the regular checking in teco_machine_main_step().
* As a sideeffect, this fixes the "Searching with large counts" test case on Mac OS
  where too large allocations were not detected as expected (apparently Mac OS
  happily gives out ridiculously large chunks of memory).
  Now, all platforms are guaranteed to have the same behaviour.

This is a total conversion of SciTECO to plain C (GNU C11).
The chance was taken to improve a lot of internal datastructures,
fix fundamental bugs and lay the foundations of future features.
The GTK user interface is now in an useable state!
All changes have been squashed together.

The language itself has almost not changed at all, except for:

* Detection of string terminators (usually Escape) now takes
  the string building characters into account.
  A string is only terminated outside of string building characters.
  In other words, you can now for instance write
  I^EQ[Hello$world]$
  This removes one of the last bits of shellisms which is out of
  place in SciTECO where no tokenization/lexing is performed.
  Consequently, the current termination character can also be
  escaped using ^Q/^R.
  This is used by auto completions to make sure that strings
  are inserted verbatim and without unwanted sideeffects.
* All strings can now safely contain null-characters
  (see also: 8-bit cleanliness).
  The null-character itself (^@) is not (yet) a valid SciTECO
  command, though.

An incomplete list of changes:

* We got rid of the BSD headers for RB trees and lists/queues.
  The problem with them was that they used a form of metaprogramming
  only to gain a bit of type safety. It also resulted in less
  readble code. This was a C++ desease.
  The new code avoids metaprogramming only to gain type safety.
  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle
  (https://github.com/jstimpfle/rb3ptr).
  This implementation is also more memory efficient than BSD's.
  The BSD list.h and queue.h has been replaced with a custom
  src/list.h.
* Fixed crashes, performance issues and compatibility issues with
  the Gtk 3 User Interface.
  It is now more or less ready for general use.
  The GDK lock is no longer used to avoid using deprecated functions.
  On the downside, the new implementation (driving the Gtk event loop
  stepwise) is even slower than the old one.
  A few glitches remain (see TODO), but it is hoped that they will
  be resolved by the Scintilla update which will be performed soon.
* A lot of program units have been split up, so they are shorter
  and easier to maintain: core-commands.c, qreg-commands.c,
  goto-commands.c, file-utils.h.
* Parser states are simply structs of callbacks now.
  They still use a kind of polymorphy using a preprocessor trick.
  TECO_DEFINE_STATE() takes an initializer list that will be
  merged with the default list of field initializers.
  To "subclass" states, you can simply define new macros that add
  initializers to existing macros.
* Parsers no longer have a "transitions" table but the input_cb()
  may use switch-case statements.
  There are also teco_machine_main_transition_t now which can
  be used to implement simple transitions. Additionally, you
  can specify functions to execute during transitions.
  This largely avoids long switch-case-statements.
* Parsers are embeddable/reusable now, at least in parse-only mode.
  This does not currently bring any advantages but may later
  be used to write a Scintilla lexer for TECO syntax highlighting.
  Once parsers are fully embeddable, it will also be possible
  to run TECO macros in a kind of coroutine which would allow
  them to process string arguments in real time.
* undo.[ch] still uses metaprogramming extensively but via
  the C preprocessor of course. On the downside, most undo
  token generators must be initiated explicitly (theoretically
  we could have used embedded functions / trampolines to
  instantiate automatically but this has turned out to be
  dangereous).
  There is a TECO_DEFINE_UNDO_CALL() to generate closures for
  arbitrary functions now (ie. to call an arbitrary function
  at undo-time). This simplified a lot of code and is much
  shorter than manually pushing undo tokens in many cases.
* Instead of the ridiculous C++ Curiously Recurring Template
  Pattern to achieve static polymorphy for user interface
  implementations, we now simply declare all functions to
  implement in interface.h and link in the implementations.
  This is possible since we no longer hace to define
  interface subclasses (all state is static variables in
  the interface's *.c files).
* Headers are now significantly shorter than in C++ since
  we can often hide more of our "class" implementations.
* Memory counting is based on dlmalloc for most platforms now.
  Unfortunately, there is no malloc implementation that
  provides an efficient constant-time memory counter that
  is guaranteed to decrease when freeing memory.
  But since we use a defined malloc implementation now,
  malloc_usable_size() can be used safely for tracking memory use.
  malloc() replacement is very tricky on Windows, so we
  use a poll thread on Windows. This can also be enabled
  on other supported platforms using --disable-malloc-replacement.
  All in all, I'm still not pleased with the state of memory
  limiting. It is a mess.
* Error handling uses GError now. This has the advantage that
  the GError codes can be reused once we support error catching
  in the SciTECO language.
* Added a few more test suite cases.
* Haiku is no longer supported as builds are instable and
  I did not manage to debug them - quite possibly Haiku bugs
  were responsible.
* Glib v2.44 or later are now required.
  The GTK UI requires Gtk+ v3.12 or later now.
  The GtkFlowBox fallback and sciteco-wrapper workaround are
  no longer required.
* We now extensively use the GCC/Clang-specific g_auto
  feature (automatic deallocations when leaving the current
  code block).
* Updated copyright to 2021.
  SciTECO has been in continuous development, even though there
  have been no commits since 2018.
* Since these changes are so significant, the target release has
  been set to v2.0.
  It is planned that beginning with v3.0, the language will be
  kept stable.

 * it turned out to be possible to provoke memory_usage
   overflows or underruns, resulting in unrecoverable states
 * a possible reason can be that at least with G++ 5.4.0,
   the compiler would sometimes call the (default) non-sized
   delete followed by our custom sized delete/deallocator.
 * This was true even after compiling Scintilla with -fsized-deallocation.
 * therefore we provide an empty non-sized delete now.
 * memory_usage counting can now be debugged by uncommenting
   DEBUG_MAGIC in memory.cpp. This uses a magic value to detect
   instrumented allocations being mixed with non-instrumented
   allocations.
 * simplified the global sized-deallocation functions
   (they are identical to the Object-class allocators).

 * largely reverts 39cfc573, but leaves in minor and documentation
   changes.
 * further experimentation of memory limiting using malloc() wrapping
   has shown additional problems, like dlsym() calling malloc-functions,
   further reducing the implementation to glibc-specific means.
   This means there had been no implementation for FreeBSD and checks
   would have to rely on undocumented internal implementation details
   of different libcs, which is not a good thing.
   * Other problems have been identified, like having to wrap calloc(),
     guarding against underruns and multi-thread safety had been identified
     but could be worked around.
 * A technique by calculating the memory usage as sbrk(0) - &end
   has been shown to be effective enough, at least on glibc.
   However even on glibc it has shortcomings since malloc() will
   somtimes use mmap() for allocations and the technique
   relies on implementation details of the libc.
   Furthermore another malloc_trim(0) had to be added to the error
   recovery in interactive mode, since glibc does not adjust the program break
   automatically (to avoid syscalls I presume).
 * On FreeBSD/jemalloc, the sbrk(0) method totally fails because jemalloc
   exclusively allocates via mmap() -> that solution was discarded as well.
 * Since all evaluated techniques turn out to be highly platform
   specific, I reverted to the simple and stable platform-specific
   mallinfo() API on Linux.
 * On FreeBSD/jemalloc, it's possible to use mallctl("stats.allocated")
   for the same purpose - so it works there, too now.
   It's slower than the other techniques, though.
 * A lengthy discussion has been added to memory.cpp, so that we
   do not repeat the previous mistakes.