Add peeking_map_while

[devins2518]

This is a continuation of my rejected PR to the stdlib for itertools. The peeking_map_while functions and associated functions are similar to peeking_take_while except this maps a Option<&I::Item> to Option<B>. This of course brings the benefit that the iterator is not advanced when the predicate returns None.

[Niedzwiedzw]

I want this badly...