Rework TH instances context

[stevladimir]

Instead of deriving instances for all datatype variables iterate over constructors generating instances based on how variables are used. Does the right thing in cases like

• Phantom type variables
  Does not add corresponding instance to the context for unused variables
• Type families
  Instead of generating (ToJSON a) for data Foo a = Foo (SomeTypeFamily a) generates ToJSON (SomeTypeFamily a)

Note that it slightly changes the behavior in cases like

data Foo a = Foo a deriving (ToJSON)

data Bar a = Bar (Foo a)
deriveToJSON defaultOptions ''Bar

will result in ToJSON (Foo a) constraint rather than ToJSON a (current behavior). I believe the former is more precise as that is exactly what generated encoder wants. The small downside is requirement for FlexibleContext extension.

TODO and open questions:

• How to test TH?
  Are there any helpers to get easy access to instance context. I think it would be nice to have something like

$(getContext $ deriveToJSON ...) `shouldBe` "ToJSON a, ToJSON b"

Can add if required. But maybe there are already established approach to test that.

• Which test cases to add?
  I sketched up some cases, but that list is not exhaustive. And they should be reworked according to previous point
• Should we test both From and To classes?
• Add Changelog