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>