Experimental flag -fdicts-cheap

This experimental flag, -fdicts-cheap, makes a let-binding that bind a
value of dictionary type look cheap.  That in turn leads to more
eta expansion.  Instead of
	f = /\a. \(d1:Ord a). let d2:Ord [a] = dfOrd a d1 in
                 \(x:a). <stuff>
which has arity 1, you get
	f = /\a. \(d1:Ord a). \(x:a).
	         let d2:Ord [a] = dfOrd a d1 in <stuff>
Now f has arity 2.

This can cretainly waste dictionary-construction work, if f is
partially applied to its dictionary argument.  However it has knock-on
effects.  Because f has arity 2, we won't float (f Int d) out of
	\x. h (f Int d)
Floating f out of this lambda makes it impossible for an h/f fusion
rule to fire; and this unexpected loss of RULE application was the
immediate reason for implementing this flag. (Roman Leshchinskiy came
across this when working on array fusion.)

I've implemented the change only in CoreUtils.arityType, which
only affects eta expansion.  I thought of putting the change in
exprIsCheap, which is a more systematic place (the former calls
the latter) but

	a) I wanted this under flag control, and the flags
	are not readily available to all callers of exprIsCheap

	b) I'm not 100% convinced that this change is a good
	idea, so it's reasonable to do the narrowest change
	that solves the immediate problem.