Exploit the 1-shot lambda HACK in etaExpandArity

We often find code like

	f :: Int -> IO ()
	f = \ x -> case ... of
			p1 -> \s -> ...rhs1...
			p2 -> \s -> ...rhs2...

where the \s is a state transformer lambda.  Almost invariably
these \s things are one-shot; that is, we virtually never say

	let
	   h = f 3
	in
	h >> h >> h

In this case we'd be much better off eta-expanding f, to

	f :: Int -> IO ()
	f = \ x \ s -> case ... of
			   p1 -> ...rhs1...
			   p2 -> ...rhs2...

GHC already has a MAJOR HACK in

	Id.isOneShotLambda

which declares that any \s::State# T is a one-shot lambda.  It's
almost always true, and it makes a big difference.


This commit simply makes use of isOneShotLambda to improve the
results of CoreUtils.etaExpandArity.  Which has the desired effect.

There isn't a flag to control the MAJOR HACK yet.  Maybe there should be.

Anyway, some of Manuel's array code should improve a lot.