Do not generate equations for monomorphic variables, during the generation of type equation from constraints.

* Simplify the workaround to compile types/boolVar.ml (it contains a variant `True and `False
  which make camlp4 go crazy)
* Regenerate the "depend" file (must be by calling make depend after
  adding new source files).

- 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

Now gamma is correctly propagated in the compilation Enviroment
and substitions are correctly applied to abstractions

* 'master' of https://git.cduce.org/cduce:
  added code for patricia trees
  Added few twisted examples

* debug-typechecking-issue-27-for-merge:
  Remove debugging code.
  Add patricia.cd file from Issue #21
  Check constraints on monomorphic variables at the right place.
  Fix a pretty printing error for ground types (the negative part was not shown due to "worth_complement" being called twice).
  Rework the type variable infrastructure. Remove it from the type structure and have the auxiliary function cache the results as needed.
  Debugging annotations.