* 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.

* 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.

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

* Greatly improved usability as a scripting language.
* The command is in DEC TECO, but in contrast to DEC TECO, we also
  support string building constructs in ^A.
* Required some refactoring: As we want it to write everything verbatim
  to stdout, the per-interface method is now teco_interface_msg_literal()
  and it has to deal with unprintable characters.
  When displaying in the UI, we use teco_curses_format_str() and TecoGtkLabel
  functions/widgets to deal with possible control codes.
* Numbers printed with `=` have to be written with a trailing linefeed,
  which would also be visible as a reverse "LF" in the UI.
  Not sure whether this is acceptable - the alternative would be to strip
  the strings before displaying them.
* Messages written to stdout are also auto-flushed at the moment.
  In the future we might want to put flushing under control of the language.
  Perhaps :^A could inhibit the flushing.

There will be a lot more commands for terminal/message
input and output soon.