Make chosen an indexed lens to see which value was traversed

[arybczak]

No description provided.

[adamgundry]

This seems reasonable to me, if slightly backwards incompatible. Should we continue to export chosen from Optics.Lens as well, since one might reasonably want it for non-indexed lenses, or at least retain a reference to chosen in the Optics.Lens docs somehow?

[arybczak]

We could export chosen from Optics.Lens, though at one point we might hit circular imports problem 🤔

On the other hand, breakage should be minimal (as usually you import umbrella modules such as Optics.Core or Optics) and there's changelog, index function on hackage and hoogle to get back on track.

[arybczak]

I added reference to chosen in Additional introduction forms of Optics.Lens.