- check pattern well-formedness in Pattern
- add arguments to exceptions in Ty and Term

(unusable so far, must eliminate match statements, too)

- do not add to the new namespace the old instantiated names
- namespaces with the same name are merged instead of raising
  an exception.

Attention: examples clash_namespace1.why, already_theory1.why,
already_theory2.why, clash_type6.why, clash_type7.why migrated 
from bench/typing/bad/ to bench/typing/good/. Please, consult
these files and make sure you comprehend and approve the new
semantics.