Module Builder

module Builder: sig .. end
Graph builders in order to persistent/imperative graphs sharing a same signature.

module type S = sig .. end
module type INT = S  with type G.V.label = int
module P: 
functor (G : Sig.P) -> S with module G = G
Persistent Graphs Builders.
module I: 
functor (G : Sig.I) -> S with module G = G
Imperative Graphs Builders.