Add generic prisms

[amesgen]

Description of the change

Generic prisms, inspired by Haskell's generic-lens, see _Ctor.

• All names are subject to bikeshedding.
• Type inference is not optimal. For example, _CC in the first example in the test suite can not be inlined (without a type annotation), but _BB can be inlined without a type signature in the second one. Maybe there are some PureScript-specific tricks (maybe some fundep stuff) to get around this, but I think this PR would be useful even with this slightly suboptimal inferability.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[MonoidMusician]

Have you seen https://github.com/LiamGoodacre/purescript-generics-rep-optics? It also provides optics based on generics-rep.

[amesgen]

Ah cool, I was not aware of that library. It seems to need a few slight changes to compile again, and has some differences (like not supporting constructors with zero or more than one argument). While I thought it would be nice to have something dual to field in profunctor-lenses, one other route would be to create a PR to https://github.com/LiamGoodacre/purescript-generics-rep-optics.