itree.c: Use `interval_tree_inherit_offset`

itree.c: Use `interval_tree_inherit_offset`

The insertion code tried to manipulate the offset in its own way,
and apparently there was a bug in it.  Replace that with a call to
`interval_tree_inherit_offset`, making the whole logic a bit simpler,
and fixing a bug along the way (not sure where the bug was, to be honest).

* src/itree.c (interval_tree_insert): Use `interval_tree_inherit_offset`.
Check the tree before insert_fix.
(recurse_check_tree): Don't check RB invariants.
(itree_limits_are_stable): Delete function (subsumed by `check_tree`).