lib: move union_find to collection

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib: introduce Queue in standard::collection

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib: introduce Stack in standard::collection

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib: introduce Heap and PriorityQueue in standard::collection

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

More standard data structures and algorithms

closes #512

Pull-Request: #554
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>