lib/ropes: Added splay method on Ropes and use it when inserting in a rope.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
tests: Adapted rope tests for splay_ropes.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge: Splay ropes

Added Splay tree operations on ropes (see [Splay tree](http://en.wikipedia.org/wiki/Splay_tree)).

Pull-Request: #532
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>