More refactoring in RnNames

More refactoring in RnNames

I rather self-indulgently spent a chunk of yesterday working on
refactoring RnNames further.  The result is significantly simpler:

* A GlobalRdrElt gets an extra field, gre_par, which records
  the parent (if any) of the name

* ImportAvails has two fields deleted: imp_env and imp_parent.
  The information provided by these fields was only used when
  processing the export list; and the same information is now readily
  generated from the GlobalRdrElts in the GlobalRdrEnv

I also did some tidying up; notably moving AvailEnv stuff from
TcRnTypes to RnNames.

The result is tha the compiler is some 130 lines shorter than before