- 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.

Fix a pretty printing error for ground types (the negative part was not shown due to "worth_complement" being called twice).

Fix a pretty printing error for ground types (the negative part was not shown due to "worth_complement" being called twice).

* new-variables-issue-26:
  Rework the type variable infrastructure. Remove it from the type structure and have the auxiliary function cache the results as needed.
  Properly check constraints on monomprhic variable during constraint generation: when a constraint 'a < t or t < 'a with 'a being monomorphic occurs during constraint generation, we check whether it holds for all 'a, that is we use plain subtyping. (It generalizes the fact that 'a < Any or Empty < 'a hold for monomorphic variables, but also accounts for constraints such as 'a < 'a | t and so on).

Conflicts:
	types/types.ml

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

when a constraint 'a < t or t < 'a with 'a being monomorphic occurs during constraint generation, we check whether
it holds for all 'a, that is we use plain subtyping.
(It generalizes the fact that 'a < Any or Empty < 'a hold for monomorphic variables, but also accounts for constraints such as 'a < 'a | t and so on).