1. Fix inverted node order, so that negative value from comparison operator
would represent lower (left) node, and positive - higher (right) node.
2. Add an argument (i.e. "context"), passed to comparison operators.
3. Change rb_tree_insert_node() to return a node - either inserted one or
already existing one.
4. Amend the interface to manipulate the actual object, instead of the
rb_node (in a similar way as Patricia-tree interface does).
5. Update all RB-tree users accordingly.
XXX: Perhaps rename rb.h to rbtree.h, since cleaning-up..
1-3 address the PR/43488 by Jeremy Huddleston.
Passes RB-tree regression tests.
Reviewed by: matt@, christos@
879d5dfb5e