trees: add `length` method to BinTree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
trees: add `is_empty` method to BinTree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
trees: add `has_key` method to BinTree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
trees: add `iterator` method to BinTree

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: trees: add `is_empty`, `has_key` and `length` methods to BinTree

Pull-Request: #470
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>