Improve pretty printing of Bdds and add debug directive to interactively inspect the internal representation of types.

Seal the representation of SortedList.Make(X).t (by making the type private). Expose Var.Set as a SortedList.S

Perform a more aggressive memoization of substituted types and register the name if the original type was named.
Make the pretty-printer re-entrant.

Fix variable ordering problem by adding a 'kind' field inside variables so that variables of the function type are processed before arguments types and after Gamma.

This reverts commit c9ad3e5c.

Conflicts:
	types/var.ml
	typing/typer.ml

All problems related to the introduction of the new syntax for
constructed types

Add a global variable table, so when we call "Var.mk id" twice, the
same variable is returned and when we call "Var.fresh v" we always
return a truly globally fresh variable.

- Var identifiers are now of type U.t instead of string
- Remove TVar from ast. Polymorphic variables are just types

- New fresh variables now share the same str, but different freshness index
- Remove the function is_internal from var module

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

Preliminary work-around to only substitute by any/empty variables that were introduced by the tallying.

of sequence|non_variable|(toplevelvar ^ rest)
with special cases when non_variable or sequence are empty and toplevelvar or rest are any
(or any combination of thoses).

Add the examples of the part2 paper in a test file.

- delta is not yet used to do anything meaningful
- remove optional argument ?fresh from Var.mk (legacy not used)
- print -> pp,printf refactoring

Conflicts:
	types/var.ml

- fix one more problem with internalize
- fix variance/contravariance problem in substituterec
- cleanup

- performance improvements (add memoization)
- fix infinite loops due to recursive structures
- add apply tests (kim)

Tallying.CS.cup -> union
Tallying.CS.cap -> prod

lessgeneral m1 m2 check if the constraint set m1 is less general of m2

- type variables are now correctly parsed as patterns
  and not as expressions

- Add a new module Var that contains all the type variables
  related machinery

- Remove old functions and unit tests about BoolVar of only
  variables, since now variables are always stored associated
  with one or more kinds