[Focus] zoom into all the elements of a collection using each

[julien-truffaut]

I think this differs from the idx and ? features in that it might result in a different kind of optic, and EPTraversal. Currently the code casts the whole macro to Lens[From,To] at the end, but now we don't know what the return is necessarily going to be.

We could use the new Scala 3 transparent keyword for "white box macro" methods,