Generalize RecApply + associated refactors

[sheaf]

• Generalize RecApply to allow the type class instance to depend on the field label.
• Change Has to a class newtype to allow partial application (this involved changing the order of arguments to facilitate the most common partial application).
• Use RecApply to rewrite the Eq and NFData instances (obviating the need for custom type classes for those).
• Use RecApply to implement a Semigroup instance.
• Add a function for building a record given an overloaded constructor method, using it to write a Monoid instance.
• Remove KeyDoesNotAppear constraint from unsafeRCons (#29).
• Remove several redundant proxy arguments by enabling AllowAmbiguousTypes.

[sheaf]

Thanks; I'm still in the process of figuring out how to use the library.

I just added a constrained traversal as I found that very useful with the generic-lens library. In this case it gives a way of traversing the fields of a record with a type-changing update:

traverseC
  :: ( TraversalC c as bs, Applicative f )
  => ( forall (l :: Symbol) a b. (KnownSymbol l, c l a b) => FldProxy l -> a -> f b )
  -> Rec as -> f ( Rec bs )

i.e. at field label l, use the typeclass instance c l a b to perform a type-changing update of type a -> f b.

This is quite general and can be used to write zipWith-style functions for instance.

I still need to test some of this a bit more though.