* src/itree.c: Use more uniform names starting with `itree_`

* src/itree.c: Use more uniform names starting with `itree_`

(struct itree_stack, itree_stack_create, itree_stack_destroy)
(itree_stack_clear, itree_stack_push_flagged, interval_stack_push)
(itree_stack_pop): Rename from `interval_stack*`.
(itree_max_height, itree_update_limit, itree_inherit_offset)
(itree_propagate_limit, itree_validate, itree_init)
(itree_rotate_left, itree_rotate_right, itree_insert_fix)
(itree_contains, itree_subtree_min, itree_remove_fix)
(itree_replace_child, itree_transplant): Rename from `interval_tree_*`.
(itree_insert_node): Rename from `interval_tree_insert`.
(itree_node_intersects): Rename from `interval_node_insert`.