Fix a grotesque bug in the HsCore.pprUfExpr
The expression

	((\x -> f x y) a b)

printed as

	(\x -> f x y a b)

which in turn caused any importing module to
behave most oddly.

This didn't show up before, because such expressions don't
happen much (they are simplified).  But one showed up in a
RULE (which happens to be simplified only very gently), and
that tickled this bug.