Add core:container/rbtree

Add a red-black tree with configurable $Key and $Value.
Also includes tests that verify it maintains RB invariants, doesn't leak.

Originally based on the CC0 implementation from literateprograms.org.
But reworked to the same API used by @Yawning's excellent `core:container/avl` for ease of use.

core/container/avl/avl.odin

@@ -5,13 +5,10 @@ The implementation is non-intrusive, and non-recursive.
 */
 package container_avl
 
-import "base:intrinsics"
-import "base:runtime"
+@(require) import "base:intrinsics"
+@(require) import "base:runtime"
 import "core:slice"
 
-_ :: intrinsics
-_ :: runtime
-
 // Originally based on the CC0 implementation by Eric Biggers
 // See: https://github.com/ebiggers/avl_tree/
 
@@ -675,4 +672,4 @@ iterator_first :: proc "contextless" (it: ^Iterator($Value)) {
 	if it._cur != nil {
 		it._next = node_next_or_prev_in_order(it._cur, it._direction)
 	}
-}
+}