itree.[ch]: Add sanity checks, comments, and minor tweaks

* src/alloc.c (mark_overlay): Add sanity check.

* src/buffer.c (next_overlay_change, previous_overlay_change):
Tweak code to keep the same vars for the bounds.

* src/itree.c (interval_tree_clear, interval_tree_insert)
(interval_tree_remove, interval_tree_insert_fix, interval_tree_remove_fix):
Adjust to the `color` -> `red` change.
(interval_tree_clear): Prefer `true/false` for booleans.
(interval_generator_create): Use an actual `interval_tree_order` value
rather than 0.
(interval_generator_next): Simplify a tiny bit.  Add comment.
(interval_generator_narrow): Add sanity check.

* src/itree.h (struct interval_node): Replace `color` field with
boolean `red` field.
(enum interval_tree_order): Remove unused `ITREE_DEFLT_ORDER` value.

* src/pdumper.c (dump_interval_node): Adjust to the
`color` -> `red` change.

1 files changed, 3 insertions, 4 deletions

diff --git a/src/itree.h b/src/itree.h
index e5d68fbfabb..8f0454dc7a4 100644
--- a/src/itree.h
+++ b/src/itree.h
@@ -45,8 +45,8 @@ struct interval_node
   ptrdiff_t offset;             /* The amount of shift to apply to this subtree. */
   uintmax_t otick;              /* offset modified tick */
   Lisp_Object data;             /* Exclusively used by the client. */
-  enum { ITREE_RED, ITREE_BLACK } color;
-  bool_bf visited : 1;          /* For traversal via generator. */
+  bool_bf red : 1;
+  bool_bf visited : 1;          /* Internal to `interval_generator_next`.  */
   bool_bf rear_advance : 1;     /* Same as for marker and overlays.  */
   bool_bf front_advance : 1;    /* Same as for marker and overlays.  */
 };
@@ -64,8 +64,7 @@ struct interval_tree
 };
 
 enum interval_tree_order {
-  ITREE_ASCENDING = 0,
-  ITREE_DEFLT_ORDER = 0,
+  ITREE_ASCENDING,
   ITREE_DESCENDING,
   ITREE_PRE_ORDER,
 };