multiple NatMap module instances

[vzaliva]

We instantiate FMap with natural numbers as a key several time:

$ ag FMapAVL.Make                                       12:41:16
ml/extracted/Memory.ml
438:  module IM = FMapAVL.Make(Z_as_OT)

coq/SigmaHCOL/SigmaHCOLRewriting.v
41:Module NM := FMapAVL.Make(Nat_as_OT).

coq/DSigmaHCOL/TypeSig.v
45:Module TM := FMapAVL.Make(Nat_as_OT).

coq/MSigmaHCOL/Memory.v
50:Module NM := FMapAVL.Make(Nat_as_OT).
~/s/helix (type-signatures|…) $                    

Perhaps this is unecessary as all the module instances are typed by the key only, and do not depend on the lement type.

Pehaps we should create Util/NatMap as use it everywhere.