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.