This patch implements three new features:
* view patterns (syntax: expression -> pat in a pattern)
* working versions of record wildcards and record puns
See the manual for detailed descriptions.

Other minor observable changes:
* There is a check prohibiting local fixity declarations
  when the variable being fixed is not defined in the same let
* The warn-unused-binds option now reports warnings for do and mdo stmts

Implementation notes: 

* The pattern renamer is now in its own module, RnPat, and the
implementation is now in a CPS style so that the correct context is
delivered to pattern expressions.

* These features required a fairly major upheaval to the renamer.
Whereas the old version used to collect up all the bindings from a let
(or top-level, or recursive do statement, ...) and put them into scope
before renaming anything, the new version does the collection as it
renames.  This allows us to do the right thing with record wildcard
patterns (which...