Overlays as an AA-tree (again)

[Joakim Jalap]

Hello Emacs devs, and happy holidays!

As ever I've been working (slowly) on the overlays-as-an-aa-tree idea,
and now I've got something that at least starts up!

So if anyone's interested, they can check out the code at
https://github.com/jockej/emacs-mirror1. The branch is
arne-without-parent. When building please build with
-DCHECK_OVERLAY_TREE for lots and lots of extra checks.

It is of course far from done, but if someone could try it out a bit I'd
be most grateful. Expect horrible breakage ;)

Happy Hacking

-- Joakim