Add future_pvec() analogue to parallel::pvec()

Not sure how I missed `parallel::pvec()`. Should add `future_pvec()` to **future.apply**.

Start by reading the R `parallel::pvec()` documentation and locating the corresponding apply entry points in future.apply. Compare the intended `future_pvec()` interface and behavior with `parallel::pvec()`; done means the new analogue is available in the package and its behavior is covered by relevant tests.

Written by the indexing model from the issue text.