Re-engineer the transition from Core to STG syntax.  Main changes in
  this commit:

    - a new pass, CoreSat, handles saturation of constructors and PrimOps,
      and puts the syntax into STG-like normal form (applications to atoms
      only, etc), modulo type applications and Notes.

    - CoreToStg is now done at the same time as StgVarInfo.  Most of the
      contents of StgVarInfo.lhs have been copied into CoreToStg.lhs and
      some simplifications made.

  less major changes:

    - globalisation of names for the purposes of object splitting is
      now done by the C code generator (which is the Right Place in
      principle, but it was a bit fiddly).

    - CoreTidy now does cloning of local binders and collection of arity
      info.  The IdInfo from CoreTidy is now *almost* the final IdInfo we
      put in the interface file, except for CafInfo.  I'm going to move
      the CafInfo collection into CoreTidy in due course too.

    - and some other minor tidyups while I was in cluster-bomb commit mode.