Expunge ThFake, cure Trac #2632

Expunge ThFake, cure Trac #2632

This patch fixes a dirty hack (the fake ThFake module), which in turn
was causing Trac #2632.

The new scheme is that the top-level binders in a TH [d| ... |] decl splice
get Internal names.  That breaks a previous invariant that things like
TyCons always have External names, but these TyCons are never long-lived;
they live only long enough to typecheck the TH quotation; the result is
discarded.  So it seems cool.

Nevertheless -- Template Haskell folk: please test your code.  The testsuite
is OK but it's conceivable that I've broken something in TH.  Let's see.