Revert "mark_overlays: Use the normal ITREE_FOREACH"

Revert "mark_overlays: Use the normal ITREE_FOREACH"

This reverts commit b8fbd42f0a7caa4cd9e2d50dd4e4b2101ac78acd,
with edits.

* src/alloc.c (mark_overlays): restore function.
(mark_buffer): Call it, not ITREE_FOREACH.
(garbage_collect): eassert (!itree_busy_p ()).
* src/itree.h: Comment tweak: explain why GC is considered risky.  It
isn't that GC itself is risky, it is that GC can call ELisp by way of
a hook, and running ELisp during iteration is risks nested iteration.