Rework the type variable infrastructure. Remove it from the type structure and...

Rework the type variable infrastructure. Remove it from the type structure and have the auxiliary function cache the results as needed.

types/var.ml

-
 module V = struct
   type t = { id : string; repr : string }
   let dump ppf t =

types/var.mli

@@ -30,6 +30,7 @@ module Set : sig
   val cardinal : t -> int
   val from_list : var list -> t
   val fold : ('a -> var -> 'a) -> 'a -> t -> 'a
+  val choose : t -> var
 end
 
 type 'a pairvar = [ `Atm of 'a | var ]