Some rearrangements that Simon & I have been working on recently:

    - CoreSat is now CorePrep, and is a general "prepare-for-code-
      generation" pass.  It does cloning, saturation of constructors &
      primops, A-normal form, and a couple of other minor fiddlings.

    - CoreTidy no longer does cloning, and minor fiddlings.  It doesn't
      need the unique supply any more, so that's removed.

    - CoreToStg now collects CafInfo and the list of CafRefs for each
      binding.  The SRT pass is much simpler now.

    - IdInfo now has a CgInfo field for "code generator info".  It currently
      contains arity (the actual code gen arity which affects the calling
      convention as opposed to the ArityInfo which is a measure of how
      many arguments the Id can be applied to before it does any work), and
      CafInfo.

      Previously we overloaded the ArityInfo field to contain both
      codegen arity and simplifier arity.  Things are cleaner now.

    - CgInfo is collected by CoreToStg, and passed back into CoreTidy in
      a loop.  The compiler will complain rather than going into a black
      hole if the CgInfo is pulled on too early.

    - Worker info in an interface file now comes with arity info attached.
      Previously the main arity info was overloaded for this purpose, but
      it lead to a few hacks in the compiler, this tidies things up somewhat.

Bottom line: we removed several fragilities, and tidied up a number of
things.  Code size should be smaller, but we'll see...