[Focus] Non-case class getters

[julien-truffaut]

There's no reason not to allow plain old getter methods on non-case classes, generating a Getter. This is something that a user might reasonably guess is supported, and it seems unreasonably to make them bust out the Optics Family documentation to do it.

eg Focus[String](_.length) // Getter[String,Int]

Original issue: optics-dev/Monocly#39