rbtree: Simplify _RBTree_Rotate()

Add and use _RBTree_Direction().
rbtree: Simplify insert and extract

Simplify _RBTree_Insert() and _RBTree_Extract().  Remove more
superfluous NULL pointer checks.  Change _RBTree_Is_root() to use only
the node.  Add parent parameter to _RBTree_Sibling().  Delete
_RBTree_Grandparent() and _RBTree_Parent_sibling().
rbtree: Simplify _RBTree_Extract()