Type signatures and skolem constants (again)
		Merge to STABLE

This commit lays to rest the vexed question of skolem constants
and type signatures.  My fix last week made type-signature variables
into ordinary meta type variables, because they can be unified
together (see Note [Signature skolems] in TcType).  But that was wrong
becuase GADTs will only refine skolems.

So this commit extends TcTyVarDetails with a new constructors, SigSkolTv,
which is a skolem (like SkolemTv) but is unifiable (like MetaTv).  It's
a bit of a hack, but the code came out quite nicely.

Now the GADT tests work.