lib: introduce abstract_tree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
trees: introduce Binary Search Tree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
trees: introduce Red Black Tree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
trees: create trees group

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: Trees

Some tree structures (needed by @xymus and @R4PaSs)

Pull-Request: #441
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>