Do a better job of eta expansion.

This showed up in one of Manuel's programs, where he got code like:

    $wsimpleGen
	     ww
	     (\ i :: Int ->
		  case i of wild1 { I# i# ->
		  case w of wild2 { I# e# ->
		  __coerce (ST RealWorld ())
		  (\ s# :: (State# RealWorld) ->
		       case writeIntArray# @ RealWorld mba# i# e# s#
		       of s2#1 { __DEFAULT ->
		       (# s2#1, () #)
		       })
		  }
		  })
	     s2#

The argument wasn't eta expanded, so it got right through to
the code generator as two separte lambdas.

Needless to say, I fiddled around with things in a vain attempt
to tidy them up.  Yell if anything seems to go wrong, or perfomance
drops on any programs.