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

[TESTS][LAMBDA] Add a test on 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

[TESTS][LAMBDA] Improved pp_lambda; update a test

- 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 !!!