[project @ 1999-03-15 15:11:03 by simonpj]

Make clear in HsType whether a for-all is explicit
in the source program or not.  Implicit for-alls now
look like 
	HsForAllTy Nothing ctxt ty
while explicit ones look like
	HsForAllTy (Just tvs) ctxt ty

Before this, the scope analysis stuff in RnSource was
actually wrong (not that anyone had noticed), but Alex Ferguson
did notice a bogus (sort-of-duplicate) error message on types
like
	f :: Eq a => Int -> Int
which led me to spot the deeper problem.  Anyway, it's all 
cool now.