Make use of GADTs rather than TypeFamilies for nicer inference and type errors

[yairchu]

We currently use the GetHyperType type family to unwrap 'AHyperTypes, but Haskell doesn't currently understand that it's a bijective type family so it doesn't infer that trees are always of shape k0 (`HyperType k1) and this leads to less helpful type errors and sometimes requires extra type signatures or usage of the asHyper inference-assisting identity function.

Using GADTs instead of TypeFamilies would provide better error messages. But transitioning to them is problematic because:

• Generic derivation for GADTs doesn't work
• lens TH code doesn't always work for GADTs
• Our own TH derivations need to be augmented to support GADTs

[expipiplus1]

FWIW kind-generics supports deriving a Generic-like interface for GADT's