Back end changes [CgExpr, ClosureInfo, CoreSat, CoreUtils,
~~~~~~~~~~~~~~~~  CmdLineOpts, HscMain, CoreToStg, StgSyn]
* Move CoreTidy and interface-file dumping *before* CoreSat.
  In this way interface files are not in A-normal form, so
  they are less bulky, and a bit easier to use as input to
  the optimiser.  

  So now CoreSat is regarded as a pre-pass to CoreToStg.
  Since CoreTidy pins on utterly-final IdInfo, CoreSat has to
  be very careful not to change the arity of any function.

* CoreSat uses OrdList instead of lists to collect floating binds
  This in turn meant I could simplify the FloatingBind type a bit

* Greatly simplfy the StgBinderInfo data type.  It was 
  gathering far more information than we needed.

* Add a flag -fkeep-stg-types, which keeps type abstractions
  and applications in STG code, for the benefit of code generators
  that are typed; notably the .NET ILX code generator.