New syntax for GADT-style record declarations, and associated refactoring

The main purpose of this patch is to fix Trac #3306, by fleshing out the
syntax for GADT-style record declraations so that you have a context in 
the type.  The new form is
   data T a where
     MkT :: forall a. Eq a => { x,y :: !a } -> T a
See discussion on the Trac ticket.

The old form is still allowed, but give a deprecation warning.

When we remove the old form we'll also get rid of the one reduce/reduce
error in the grammar. Hurrah!

While I was at it, I failed as usual to resist the temptation to do lots of
refactoring.  The parsing of data/type declarations is now much simpler and
more uniform.  Less code, less chance of errors, and more functionality.
Took longer than I planned, though.

ConDecl has record syntax, but it was not being used consistently, so I
pushed that through the compiler.

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