Retool to use purrr as backend?

[ramhiser]

As discussed in #46, having the iterators package as a backend for itertools2 yields slow performance in a few cases. On the other hand, @hadley is on a functional-programming kick (a good thing). Besides, the purrr package is purrrdy sweet and fits the more modern R workflow.

Explore retooling purrr as a backend.

[ramhiser]

This goes along with #43.