Ajoute le module Tas pour remplacer la fonction List.sort.

Mise a jour la fonction eval_star et la fonction eval_qtree (remplace List.sort)
Dans le module Tas il y a des fonctions ci-dessous:
1) val size : 'a tas -> int
2) val height : 'a tas -> int
3) val equilibre : 'a tas -> bool
4) val is_tas : int tas -> bool   ------- il faut ameliorer
5) val pop : Naive_tree.t -> Naive_tree.node tas -> Naive_tree.node
6) val push : Naive_tree.t -> Naive_tree.node tas -> Naive_tree.node -> Naive_tree.node tas
7) val tas_of_list : Naive_tree.t -> Naive_tree.node list -> Naive_tree.node tas
8) val list_of_tas : Naive_tree.t -> Naive_tree.node tas -> Naive_tree.node list
9) val is_empty : Naive_tree.node tas ->bool
10) val sort_of_list : Naive_tree.t -> Naive_tree.node list -> Naive_tree.node list