Add iterator.cycle

[irh]

Since #99 iterators have been copyable, so it should be possible now to implement an iterator.cycle iterator adaptor that copies an iterator and repeats its output endlessly.

(1, 2, 3).cycle().take(10).to_list()
# [1, 2, 3, 1, 2, 3, 1, 2, 3, 1]

[alisomay]

Hey this is cool.
There is an attempt of an implementation here when you have time to take a look.
#117

[irh]

Implemented in #117.