Add a MapView type providing lazy Map operations

[julienrf]

Similar to IndexedView[A], we might want to add a MapView[K, V] collection type providing map-like transformation operations.

We have to figure out how the View hierarchy should coexist with the Iterable hierarchy.

[lrytz]

There's also SeqView. In 2.12, List(1,2).view.reverse works (https://github.com/scala/scala/blob/2.13.x/test/files/run/ReverseSeqView.scala)

[julienrf]

@lrytz For reverse I’m wondering whether we should support it at all in SeqView. It is possible to delay the process of reversing the underlying collection, but that process will reverse the whole collection at once (it can not be fused with other intermediate transformations).