Do not allow Typeable on constraints (Trac #9858)

The astonishingly-ingenious trio of
Shachaf Ben-Kiki, Ørjan Johansen and Nathan van Doorn
managed to persuade GHC 7.10.1 to cough up unsafeCoerce.

That is very bad. This patch fixes it by no allowing Typable
on Constraint-kinded things.  And that seems right, since
it is, in effect, a form of impredicative polymorphism,
which Typeable definitely doesn't support.

We may want to creep back in the direction of allowing
Typeable on constraints one day, but this is a good
fix for now, and closes a terrible hole.