itree.c: Fix corner case errors in offsets

itree.c: Fix corner case errors in offsets

In some cases, `interval_tree_remove` could cause some nodes to
inherit fewer (or additional) offsets than the should because nodes
were transplanted between two parts of the tree where offsets had not
been propagated "equally".  So we remove/apply all offsets along the
path between the two points of a transplant before doing the transplant.

* src/itree.c (interval_tree_subtree_min): Move before first use; delete
the declaration; add an `otick` argument, and use it to update offsets
along the way.
(interval_tree_remove): Update all offsets on the way from `node` to `min`.
Reorder some of the operations so that when we transplant `min` to `node`
those nodes are in the proper state where `interval_tree_transplant`
can do its sanity checks.
(itree_total_offset): New function.
(interval_tree_transplant): Use it to sanity check that improper
offsets aren't accidentally inherited/lost because of the transplant.
(itree_newlimit): New function.
(itree_limit_is_stable, interval_tree_update_limit)
(interval_tree_propagate_limit): Use it.
(null_is_sane): Remove `inline` annotation; it's not needed.
(interval_tree_inherit_offset): Sanity check that `offset` is 0 when
`otick` is uptodate.  Skip the unneeded increments when the offset is 0.
(interval_tree_insert_fix): Add sanity check that we indeed have 2 reds.