This adds back in the patch 
  * UNDO: Be a little keener to inline

It originally broke the compiler because it tickled a Cmm optimisation bug,
now fixed.  

In revisiting this I have also make inlining a bit cleverer, in response to
more examples from Roman. In particular

  * CoreUnfold.CallCtxt is a data type that tells something about
    the context of a call.  The new feature is that if the context is
    the argument position of a function call, we record both 
	- whether the function (or some higher up function) has rules
	- what the argument discount in that position is
    Either of these make functions keener to inline, even if it's
    in a lazy position

  * There was conseqential tidying up on the data type of CallCont.
    In particular I got rid of the now-unused LetRhsFlag