Field syntax for optics

[julien-truffaut]

Add a field(lambda) on all Optics such that:

case class User(name: String, address: Address)
case class Address(streeetNumber: Int, postcode: String)

Focus[User]()
  .field(_.address)
  .field(_.streetNumber) == Focus[User](_.address.streetNumber)

It is a bit redundant but this method is meant for updating existing optics:

val user: Lens[State, User] = ... // suppose we have it defined

user.field(_.name) == user.andThen(Focus[User](_.name))

[julien-truffaut]

This duplicate code from GenLens macro, I couldn't find out how to pass a Boolean flag ...

I don't think it matters too much since it is for Scala 2 and we don't plan to touch macro further in this version of Scala.

[julien-truffaut]

need more discussion, closing for now