MPS experiment successful

[Gerd Möllmann]

If someone doesn't remember what MPS is about: concurrent, incremental,
generational, mostly-copying, thread-safe GC.

I've now reached the point that I wanted to reach to consider my
experiment of using MPS in Emacs successful.

- Emacs builds from scratch
- Tests succeed to a sufficient degree
- Emacs starts in GUI mode

That's all. There is nothing more. And I'm currently undecided how to
proceed with this.

To make this credible :-), you can find the sources at

  https://github.com/gerd-moellmann/emacs-with-cl-packages/tree/igc

Please note that this is based on my local Emacs (see Github page). It
probably doesn't build/work on anything but macOS, using different Elisp
packages than I do, or not using the ones I do :-), and so on. I also
regularly force-push, and do other mean things.