Cleanups to ART

* Change art_compare_prefix to take a single length rather than two.
* Use art_compare_keys where possible.
* Use art_val_t directly (through an alias) rather than defining a separate
  type.
* Remove key_chunk from art_indexed_child_t.
* Use node->base where possible.
* Merge art_create_iterator into art_init_iterator.

include/roaring/art/art.h

@@ -33,7 +33,7 @@ namespace internal {
 #endif
 
 typedef uint8_t art_key_chunk_t;
-typedef void art_node_t;
+typedef struct art_node_s art_node_t;
 
 /**
  * Wrapper to allow an empty tree.
@@ -50,7 +50,7 @@ typedef struct art_s {
  * contains the key corresponding to the value.
  */
 typedef struct art_val_s {
- art_key_chunk_t _pad[ART_KEY_BYTES];
+ art_key_chunk_t key[ART_KEY_BYTES];
 } art_val_t;
 
 /**
@@ -121,15 +121,11 @@ typedef struct art_iterator_s {
 } art_iterator_t;
 
 /**
- * Creates an iterator. Must be initialized through `art_init_iterator` first.
- */
-art_iterator_t art_create_iterator();
-
-/**
- * Moves the iterator to the first or last entry in the ART, depending on
- * `first`.
+ * Creates an iterator initialzed to the first or last entry in the ART,
+ * depending on `first`. The iterator is not valid if there are no entries in
+ * the ART.
  */
-void art_init_iterator(const art_t *art, art_iterator_t *iterator, bool first);
+art_iterator_t art_init_iterator(const art_t *art, bool first);
 
 /**
  * Returns an initialized iterator positioned at a key equal to or greater than

include/roaring/roaring64.h

@@ -28,7 +28,7 @@ typedef struct roaring64_leaf_s roaring64_leaf_t;
  * contexts associated with that bitmap.
  */
 typedef struct roaring64_bulk_context_s {
- uint8_t high_bytes[ART_KEY_BYTES];
+ uint8_t high_bytes[6];
  uint16_t low_bytes;
  roaring64_leaf_t *leaf;
 } roaring64_bulk_context_t;