Improvements to record puns, wildcards

Improvements to record puns, wildcards

* Make C { A.a } work with punning, expanding to C { A.a = a }

* Make it so that, with -fwarn-unused-matches,
        f (C {..}) = x
  does not complain about the bindings introduced by the "..".

* Make -XRecordWildCards implies -XDisambiguateRecordFields.

* Overall refactoring of RnPat, which had become very crufty.
  In particular, there is now a monad, CpsRn, private to RnPat,
  which deals with the cps-style plumbing.  This is why so many
  lines of RnPat have changed.

* Refactor the treatment of renaming of record fields into two passes
	- rnHsRecFields1, used both for patterns and expressions,
	     which expands puns, wild-cards
  	- a local renamer in RnPat for fields in patterns
	- a local renamer in RnExpr for fields in construction and update
  This make it all MUCH easier to understand

* Improve documentation of record puns, wildcards, and disambiguation