Some feature, plus some refactoring

[kenbot]

Adds the some feature in the established pattern, with a parser & generator file in a new feature package, solving #17

Of note:

• Fixes the restriction we had on selecting fields with nested type parameters, like Option[A] or F[A].
• The macro signature is now transparent inline, which allows us to choose the static return type inside the macro. The declared return type is Any, which we can replace with a more civilised top type when the optics live inside the same type hierarchy.
• Extension method some available with a Focus._ import, for want of a better scheme.
• Big refactor of ParserLoop, whose flaws were much more clearly visible with the addition of a second feature.
• ParserLoop now knows nothing of the features and only performs iteration and assembly of the result.
• Special treatment of the lambda argument has been separated from the feature matching.
• FieldSelectParser now only exposes one public member, the FieldSelect pattern matching parser.
• The TypeInfo type is deleted; it turned out to just be a FieldSelect thing, there was no need for this information in every FocusAction.
• The composeOptics method in GeneratorLoop now has a case for every combination of Lens, Optional, Prism and Iso.
• Brushed up the existing tests a bit to fit the pattern of Julien's test suite for some a bit better.

[julien-truffaut]

It looks amazing, I like how you split the parsers and generators!

[julien-truffaut]

Great Job