Implement `nth` and `nth_back`

[tcharding]

Override the default implementations for the iterators in this crate.

This is an optimization thing not a functionality thing, see comments below where its explained.

[tcharding]

Don't know what I was thinking when I wrote this:

• There is no such thing as nth_back.
• The default nth works just fine.

I added some unit tests in #32 just to be sure.

[Kixunil]

Overriding is good for performance if we can seek in O(1) since nth is O(n). For nth_back check DoubleEndedIterator.

[tcharding]

ooooo, nice. Thanks man, so I should have put more context in the issue in the first place. Thanks!