Introducing OpticGet to fix up getters

[kenbot]

• Regretfully adding new type alias OpticGet[-S, +A] to correctly fix up the type for getter optics.
• GetOptic.id[A] returns an identity getter, which decapitates Modify/ReverseGet when composed with another optic
• The to method works now as you would expect, getters "just work"
• You can now do variance-aware compositions like (a: Optic[Get, Zoo, Tiger]).andThen(b: Optic[Animal, Photo])