Improve the debugging of the unify step of the tallying. Fix a bug where substitutions were applied in the wrong order, yielding partially empty types.

Refactor the Bool/BoolVar code so that they share the same interface. Give access to the underlying atom module in BoolVar.

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

Fix minor problem in runtime/value.ml

Prevent the compiler from statically allocating an persistent variant (that is mutated by the OCaml runtime)

Conflicts:
	runtime/value.ml

Conflicts:
	Makefile.distrib
	depend

Change $$ in >> in types.ml for the substitution operator

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

Partial fix for issue #20. In the Value.t type, the Concat _ value and the Pair value must have the same underlying representation (since their tag is mutated).
This actually also corrects a bug: Concat _ nodes acts as constructor (like pairs, xml, records, …) and should also be decorated with a sigma. Adding a sigma component to Concat make the segfault disapear. However, for the moment substitutions in Concat are never used.

Tallying and printing still broken.

- from Module.xxx_to_string to Module.Print.string_of_xxxx
- from Module.print_xxx to Module.Print.pp_xx

[TESTS][LAMBDA] add tests for comp of sigmas

	function so it's not working for now)
[TESTS][LAMBDA] Add chars (shows it should be easy to use the '<uident> syntax
	for type variables in CDuce)

[TESTS][LAMBDA] Update tests; improve lambda printer

Now we correctly distibguish polymorphic and monomorphic types and
set sigma substitutions accordingly

update .ocamlinit with parse_expr

- redifine sigma (Value.sigma and Lamda.sigma)
- add sigma application in eval
- fix typing rules for abstraction and match
- XXX This commit breaks all tests !!!