[Proposal] map-of index should be part of :in during a walk

[green-coder]

(defprotocol LensSchema
  (-keep [this] "returns truthy if schema contributes to value path")

I think that the index in :map-of should be part of :in during a walk as it contributes to the value path, otherwise we don't differentiate between the key and the value.

[green-coder]

Other proposal: to replace the 0 and 1 indices by something like ::m/key and ::m/val for this schema, similarly to how ::m/in is used for collections.