Add variant of ObservableArrayType.reduce that reduces over a mapping

[lorentey]

Mapping an array requires maintaining an index mapping, which is superfluous when we're going to ignore the ordering anyway; so array.map{$0.someInteger}.sum() does extra work.

[lorentey]

Ugh, not really. If the key is not observable, then reduce's add and remove can already extract it on the fly. If it is observable, then it may change on its own, so array.map{$0.someInteger}.reduce(…) is the way to go.