Implement gcaseR: functions before datatype

[fpringle]

Re-implement gcaseR, by converting gcase (which uses Chains) to a new type ChainsR. This looks
similar to the original implementation here, but without trying to do the recursion directly via ChainsR (which seems
to be impossible with the current implementation).

The important commits are:
b79ced2 - Generalise Chains to have different return types in the inner and outer chains
5a17f25 - ChainsR is basically the same as Chains, with the arguments in a different order
48058f8 - Finally we can re-implement gcaseR

Note

This is undoubtedly more ergonomic, since it allows us to use partial application nicely:

let maybeToEither err = maybeR (Left err) Right
in  ...

However, this carries a slight performance impact. It will always be faster to use 'gcase', so if performance is critical in your use-case, use that. Then again, if performance is really critical, you'll always be better off writing your analysis function manually; or just pattern-matching directly.

Note to self

Update Changelog before merging.