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Earlier we anticipated declaring projections in this way,
but never implemented it, and I still think this would
be a hack, not in line with the rest of the language.
Anyway, it's a three-line change, easily revertible.

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

- 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.

Interface consistency is god.

Update the whole shebang to use the new core modules

- assure generation of new variables on create_ls_defn

to use proposition names as propositional variables

- unify Lfunction and Lpredicate (mucho bettar)
- separation of prop and fmla
- making prop and tvsymbol private aliases of ident

without changing their order - NOT a topological sort

- split clone code between contexts and theories

- add [Dclone] declarations after cloned decls

- account for the used theories in [ctxt_known]
  (the hash-consed [Duse] declaration is referenced)

- provide use and clone for contexts with a distinct semantics:

    Context.use_export ctxt th :
        Copies the contents of [th] and its dependencies
        into [ctxt]. The [Duse] declarations are erased
        from the list of decls, but keeped in [ctxt_known],
        so that theories are not copied twice. Useful to
        build flat contexts.

    Context.clone_export ctxt th :
        Clones the contents of [th] into [ctxt]. Dependencies
        of [th] are copied, too. Erases the [Duse] declarations,
        but keeps them in [ctxt_known], so that theories are not
        copied twice. Useful to build flat contexts.

    Theory.use_export uc th :
        Marks the dependency on [th]. [Duse th] is added to [uc],
        unless [th] is already used.

    Theory.clone_export uc th :
        Clones the contents of [th] into [uc]. The dependencies
        of [th] become dependencies of [uc]. [Duse] declarations
        are added, when a theory is used for the first time.