Rename mapConcurrently and mapConcurrently_

[David Feuer]

The rest of the Haskell world calls things with types like (a -> f b) -> t a -> f (t b) traversals, rather than maps. Presumably, async has been around since before that convention developed. I propose to

1. Add traverseConcurrently and traverseConcurrently_ functions.
2. Eventually deprecate and remove mapConcurrently and mapConcurrently_.