- 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