STatus of MPS branch

[Gerd Möllmann]

I've now fixed all aborts/bus errors and so on in scratch/igc that were
caused by things related to GC, and that I could identify from running
the tests.

  SUMMARY OF TEST RESULTS
  -----------------------
  Files examined: 513
  Ran 7602 tests, 7327 results as expected, 7 unexpected, 268 skipped
  3 files contained unexpected results:
    src/emacs-module-tests.log
    lisp/progmodes/eglot-tests.log
    lisp/net/tramp-tests.log

There are many things other people could help with at this point. Some
require no knowledge of MPS, or C. For example:

- Make it work on anything not macOS.

- Run tests with native compilation.

- Determine what the reason for the remaining test failure is.

- Figure out what is done in the old GC in garbage_collect that is not
  directly related to GC, and how to do that when GC is concurrent.
  Examples are buffer undo_list, window prev_buffers/next_buffers
  handling (which I don't understand at all, at present), and whatever
  else there is in garbage_collect/sweep.

- Figure out which Elisp functionality can't be supported with igc and
  what to do with them. A trivial example would be post-gc-hook.

- Or something that requires some MPS knowleage: implement weak hash
  tables.

- and so on, and so on...

If someone wants to help with something, I think it would be a good idea
to say so here.