`nth_front()`/`nth_back()` methods

[nazar-pc]

Since there are already push/pop methods, it'd be also convenient to have getters that get nth method from the front or from the end.

Right now there is get(), but it requires more effort to use for the same purpose and .iter().nth()/.iter().rev().nth() are likely much less efficient than direct methods would be.

[andreacorbellini]

Excellent suggestion, thank you! Done in d2d2421