Add debugging help for nested iterators (nug#58144)

Add debugging help for nested iterators (nug#58144)

When starting an iteration, store __FILE__ and __LINE__ where this
happens in the interval_tree structure.

* src/buffer.h (buffer_overlay_iter_start): New macro adding __FILE
and __LINE__.
(buffer_overlay_iter_start1): Renamed from ..._start.
* src/itree.h (struct interval_tree): Add file and line info.
* src/itree.c: (interval_tree_contains, interval_tree_nodes,
interval_tree_insert_gap): Pass __FILE__ and __LINE__ to iter_start.
(interval_tree_iter_start): Record file and line info in tree.