Refocus feature

[kenbot]

• Adds extension method refocus to all optics & applied optics, which is equivalent to optic.andThen(Focus[Foo](_.....)).
• Resolves [Focus] Mix Focus and standard optics #1056
• Must be an extension method rather than a direct method on optics, so as to cleanly separate the macro code.
• Astonishingly, can treat all optics and applied optics uniformly with only one simple implementation.
• Privately defines a type AnyOptic[S,A] = Setter[S,A] | Fold[S,A] | AppliedSetter[S,A] | AppliedFold[S,A]. Should this be a public definition? Probably not, because its usage is not likely to be meaningful beyond this macro, and would be highly susceptible to change.
• Big fat test suite testing many combinations of optic types, and especially Getter and Fold, since they are special cases, because their andThen method only has one type parameter.
• Have not added a Scala 2 equivalent, corresponding to the single-field Focus functionality supported in Scala 2. It makes sense to add this, but perhaps that can be a different ticket

[julien-truffaut]

Awesome work @kenbot
For Scala 2, maybe we can resurect #1125

I don't have a strong opinion about AnyOptic (public or private)