Complete the hack that lets us give 'error' the type

	error :: forall a:?. String -> a

The kind '?' (also known as 'Type AnyBox') means that
error can be applied to unboxed types too.  unsafeCoerce is
similar.  But the 'real' type of error is:

	error :: forall bv. forall a:Type bv. String -> a

That is, it's really polymorphic in the boxity of a; that's
different to saying that the type bound to 'a' must have
kind '?'.  

The fix was in TcType.inst_tyvar, where we look for the kind '?'
and generate a boxity variable instead.  Sigh.