Add -XImpredicativeTypes, and tighten up type-validity checking (cf Trac 2019)

Add -XImpredicativeTypes, and tighten up type-validity checking (cf Trac 2019)

Somehow we didn't have a separate flag for impredicativity; now we do.

Furthermore, Trac #2019 showed up a missing test for monotypes in data
constructor return types.  And I realised that we were even allowing
things like
	Num (forall a. a) => ...
which we definitely should not.

This patch insists on monotypes in several places where we were (wrongly)
too liberal before.

Could be merged to 6.8 but no big deal.