bug#58158: 29.0.50; [overlay] Interval tree iteration considered harmful

[Gerd Möllmann]

In its current form, interval tree iteration works like this:

1. Call begin_iteration(T) to iterate over tree T
2. do stuff
3. Call end_iteration(T)

with the following rules:

- Begin_iteration and end_iteration must be paired.

- There can be only one iteration per tree at a time.  Nested iteration
  over the same tree is not supported (abort).

- No GC may happen in step 2.  This is because mark_buffer iterates over
  buffer overlays.

I think this is an exceedingly dangerous design.