Handle Functor Composition in Generic Instances

[julmb]

Second attempt at fixing #1059 after #1103 did not cover all cases. First discussion of the approach in #1059 (comment).

Tasks:

• add GOmitFromJSON and GOmitToJSON type classes and instances
• replace S1 s * instances with single generic implementation based on GOmitFromJSON and GOmitToJSON
• investigate field omission propagation in existing instances GFromJSON a (f :.: g) and GToJSON' e a (f :.: g)
• inline recordParseJSONImpl that now has just a single call site
• add regression tests

I would be grateful for feedback by people more knowledgeable about the existing infrastructure than me.

[julmb]

This PR fixes #1059 (6350e9a, c56cfc1) as well as another issue (c1f6c14) that was present but inaccessible due to the inability to derive instances imvolving (:.:). The regression tests (66d68f5) cover both of these. The remaining commits (0f36b77, 5957128) are cleanup and refactoring.