This was especially dangerous since the introduction of
a message level parameter, which could still be popped from
the expression stack in parse-only mode or during lexing.
This effectively broke n^A interactively in GTK.

* I.e. you can now log warnings and errors from SciTECO code as well.
* We do not need a version of ^A accepting code points, since this is
  supported by ^T already.

* When passing a struct that should not be modified, I usually use a const pointer.
* Strings however are small 2-word objects and they are often now already passed via separate
  `gchar*` and gsize parameters. So it is consistent to pass teco_string_t by value as well.
  A teco_string_t will usually fit into registers just like a pointer.
* It's now obvious which function just _uses_ and which function _modifies_ a string.
  There is also no chance to pass a NULL pointer to those functions.

* Requiring state callbacks by generating their names (e.g. NAME##_input) has several disadvantages:
  * The callback is not explicitly referenced when the state is defined.
    So an unintroduced reader will see some static function, which is nowhere referenced and still
    doesn't cause "unused" warnings.
  * You cannot choose the name of function that implements the callback freely.
  * In "substates" you need to generate a callback function if you want to provide a default.
    You also need to provide dummy wrapper functions whenever you want to reuse some existing
    function as the implementation.
* Instead, we are now using static assertions to check whether certain callbacks have been
  implemented.
  Unfortunately, this does not work on all compilers. In particular GCC won't consider
  references to state objects fully constant (even though they are) and does not allow
  them in _Static_assert (G_STATIC_ASSERT). This could only be made to work in newer GCC
  with -std=c2x or -std=gnu23 in combination with constexpr.
  It does work on Clang, though.
  So I introduced TECO_ASSERT_SAFE() which also passes if the expression is *not* constant.
  These static assertions are not crucial - they do not check anything that can differ between
  systems. So we can always rely on the checks performed by FreeBSD CI for instance.
  Also, you will of course quickly notice missing callbacks at runtime - with and without
  additional runtime assertions.
* All mandatory callbacks must still be explicitly initialized in the TECO_DEFINE_STATE calls.
* After getting rid of generated callback implementations, the TECO_DEFINE_STATE macros
  can finally be qualified with `static`.
* The TECO_DECLARE_STATE() macro has been removed. It no longer abstracts anything
  and cannot be used to declare static teco_state_t anyway.
  Also TECO_DEFINE_UNDO_CALL() also doesn't have a DECLARE counterpart.

* Instead of rendering a teco_string_t into a Scintilla view (GTK)
  and an ncurses window (Curses), it is now a Scintilla view and document
  that is modified directly.
* Reduces redundancies between GTK and Curses UIs.
* It will be more efficient on very large command lines, especially on GTK.
* We can now redirect Scintilla messages to the command line view in order
  to configure syntax highlighting, the margin, rubout indicator style and
  scroll behavior (TODO).
* This will also simplify the configuration of multi-line command lines (TODO).
* Since INDIC_PLAIN is not supported by Scinterm, rubbed out command lines
  are now styled with INDIC_STRAIGHTBOX (background color).

* n:^T always prints bytes (cf. :^A)
* ^T without arguments returns a codepoint or byte from stdin.
  In interactive mode, this currentply places a cursor in the message line and waits for a keypress.

In particular, fixes the test case `3<255=>` which would print
only one number in interactive mode.

* refactored some code that is common with Xq into teco_get_range_args().

* ^A uses the default code page without colon, just like ^U/EU.
  This is usually UTF8, unless you run with --8bit.
  It would make just as little sense to inherit the codepage from the
  current document.
* Ensures that code like `:^A^E<0xFF>^A` really outputs byte 0xFF.
* DEC TECO doesn't have the colon modifier, but it has a colon modifier
  for ^T to enforce raw output.
  In SciTECO, the ^T vs. :^T distinction will also be between default codepage
  and ANSI. It makes sense because ^T should treat its numeric arguments like <I>
  for consistency.