Focus macro

[julien-truffaut]

This ticket is here to track and prioritize Focus macro features:

• Single use focus pre-applied on a target using .focus(...) [Focus] Single-use with pre-applied target #1027
• zoom into a branch of an enumeration/sealed trait using as (generate a monomorphic Prism) [Focus] add as for enumerations #1028
• zoom into all the elements of a collection using each [Focus] zoom into all the elements of a collection using each #1029
• zoom into a single element of a collection using index or at [Focus] zoom into a single element of a collection using index or at #1030
• zoom into an Option using withDefault(...) [Focus] add withDefault feature #1031
• generate an Iso instead of a Lens when zooming on a case class with a single field [Focus] Generate Isos rather than Lenses for single-argument case classes #1032
• zoom into a common field of an enumeration (generate a monomorphic Lens [Focus] zoom into a common field of an enumeration #1033