lib/standard/ropes: API change, Leaves are now abstract for future flexibility.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: Concat nodes cannot be constructed without children anymore.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/ropes: Added to_leaf method on RopeNode.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/ropes_debug: Adapted new leaves for to_dot operation.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge: Rope api

Small modifications of the API that will be required for future pull requests such as Splay ropes and Bufferized ropes.

Pull-Request: #518
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>