rust: rbtree: add RBTreeCursor

Add a cursor interface to `RBTree`, supporting the following use cases:
- Inspect the current node pointed to by the cursor, inspect/move to
  it's neighbors in sort order (bidirectionally).
- Mutate the tree itself by removing the current node pointed to by the
  cursor, or one of its neighbors.

Add functions to obtain a cursor to the tree by key:
- The node with the smallest key
- The node with the largest key
- The node matching the given key, or the one with the next larger key

The cursor abstraction is needed by the binder driver to efficiently
search for nodes and (conditionally) modify them, as well as their
neighbors [1].

Link: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Tested-by: Alice Ryhl <[email protected]>
Signed-off-by: Matt Gilbride <[email protected]>