Laws for GHC.Generics

[Gershom B]

I had a bit of discussion today on how to handle generic instances for
abstract types.

It resolved to the following question, in a sense -- what are the laws
for the Generic typeclass?

Should require that `from` and `to` be an isomorphism of data types
(i.e. that `from . to` and `to . from` both be the identity) or should
we require only the weaker one-sided retract condition (i.e. that
`from . to` be the identity.). If the latter, is it better that `from`
be partial (which i  prefer) or that it quotient together "equivalent"
representations (i.e. representing a map as a `fromList` as
syb-generics do).

The documentation doesn't seem to provide clear guidance here, and it
might be worth discussing if some should be added.

Cheers,
Gershom