[Focus] Non-case class setters in?

[julien-truffaut]

Dual to #1034, conceivably someone, wishing to have write-only semantics on their immutable data model, might implement their own copy method. I can't even think of a use case for a Setter optic to be honest, I'm sure this is extremely low priority, and I can't imagine user demand leading us here.

There's a little voice in my head complaining that Setter is the only optic inaccessible to the Focus macro, but I'm not at all sure it deserves to be listened to.