monomorphism bug

I can't track this one down, so I'll post it:

	Prelude> :t let f = const 1 in let g = f in g
	let {...} in (let {...} in g) :: Num a => b -> c

The error here is that a and c should be identified.  In larger examples,
this can cause real trouble.

More detail: f is subject to the Monomorphism Restriction, so its
return type should be monomorphic.  Hugs gets this right, as seen in

	Prelude> :t let f = const 1 in (f,f)
	let {...} in (f,f) :: Num a => (b -> a,c -> a)

However it loses the constraint on the type of g, as seen in

	Prelude> :t let f = const 1 in let g :: a -> b; g = f in g
	let {...} in (let {...} in g) :: Num a => b -> c

(which it should reject).  It seems essential that the type of f contain
fully polymorphic variables as well as the constrained one.