[Focus] add as for enumerations

[julien-truffaut]

sealed trait User {
  def id: Long
}

case class Reader(id: Long, isPremium: Boolean) extends User
case class Editor(id: Long, favoriteFont: String) extends User

Focus[User](_.as[Reader])
// res: Prism[User, Reader]

Focus[User](_.as[Reader].isPremium)
// res: Optional[User, Boolean]

Operator to generate a Prism based on a type-cast.

I'm pretty sure that any type-cast gives us a Prism, so we shouldn't need to limit it to sealed hierarchies or enums.

[kenbot]

Given that this is a useful enough feature to have a privileged syntax in Focus, it's worth considering whether it should also be a Monocle operator satisfying Focus[Foo](_.bar.as[Baz]) == Focus[Foo](_.bar).as[Baz]