Add several new record features

Add several new record features

1. Record disambiguation (-fdisambiguate-record-fields)

In record construction and pattern matching (although not
in record updates) it is clear which field name is intended
even if there are several in scope.  This extension uses
the constructor to disambiguate.  Thus
	C { x=3 }
uses the 'x' field from constructor C (assuming there is one)
even if there are many x's in scope.

2. Record punning (-frecord-puns)

In a record construction or pattern match or update you can
omit the "=" part, thus
	C { x, y }
This is just syntactic sugar for
	C { x=x, y=y }

3.  Dot-dot notation for records (-frecord-dot-dot)

In record construction or pattern match (but not update)
you can use ".." to mean "all the remaining fields".  So

	C { x=v, .. }

means to fill in the remaining fields to give

	C { x=v, y=y }

(assuming C has fields x and y).  This might reasonably
considered very dodgy stuff.  For pattern-matching it brings
into scope a bunch of things that are not explictly mentioned;
and in record construction it just picks whatver 'y' is in
scope for the 'y' field.   Still, Lennart Augustsson really
wants it, and it's a feature that is extremely easy to explain.

Implementation
~~~~~~~~~~~~~~
I thought of using the "parent" field in the GlobalRdrEnv, but
that's really used for import/export and just isn't right for this.
For example, for import/export a field is a subordinate of the *type
constructor* whereas here we need to know what fields belong to a
particular *data* constructor.

The main thing is that we need to map a data constructor to its
fields, and we need to do so in the renamer.   For imported modules
it's easy: just look in the imported TypeEnv.  For the module being
compiled, we make a new field tcg_field_env in the TcGblEnv.
The important functions are
	RnEnv.lookupRecordBndr
	RnEnv.lookupConstructorFields

There is still a significant infelicity in the way the renamer
works on patterns, which I'll tackle next.

I also did quite a bit of refactoring in the representation of
record fields (mainly in HsPat).***END OF DESCRIPTION***

Place the long patch description above the ***END OF DESCRIPTION*** marker.
The first line of this file will be the patch name.

This patch contains the following changes:

M ./compiler/deSugar/Check.lhs -3 +5
M ./compiler/deSugar/Coverage.lhs -6 +7
M ./compiler/deSugar/DsExpr.lhs -6 +13
M ./compiler/deSugar/DsMeta.hs -8 +8
M ./compiler/deSugar/DsUtils.lhs -1 +1
M ./compiler/deSugar/MatchCon.lhs -2 +2
M ./compiler/hsSyn/Convert.lhs -3 +3
M ./compiler/hsSyn/HsDecls.lhs -9 +25
M ./compiler/hsSyn/HsExpr.lhs -13 +3
M ./compiler/hsSyn/HsPat.lhs -25 +63
M ./compiler/hsSyn/HsUtils.lhs -3 +3
M ./compiler/main/DynFlags.hs +6
M ./compiler/parser/Parser.y.pp -13 +17
M ./compiler/parser/RdrHsSyn.lhs -16 +18
M ./compiler/rename/RnBinds.lhs -2 +2
M ./compiler/rename/RnEnv.lhs -22 +82
M ./compiler/rename/RnExpr.lhs -34 +12
M ./compiler/rename/RnHsSyn.lhs -3 +2
M ./compiler/rename/RnSource.lhs -50 +78
M ./compiler/rename/RnTypes.lhs -50 +84
M ./compiler/typecheck/TcExpr.lhs -18 +18
M ./compiler/typecheck/TcHsSyn.lhs -20 +21
M ./compiler/typecheck/TcPat.lhs -8 +6
M ./compiler/typecheck/TcRnMonad.lhs -6 +15
M ./compiler/typecheck/TcRnTypes.lhs -2 +11
M ./compiler/typecheck/TcTyClsDecls.lhs -3 +4
M ./docs/users_guide/flags.xml +7
M ./docs/users_guide/glasgow_exts.xml +42