Reimplement some built-ins in terms of the standard library.

As a result:

  * functions such as `nth` should work correctly on iterators;
  * `nth` will raise `IndexError` (in a fashion consistent with `get`)
    when the index is out of bounds;
  * `take`, etc. will raise `ValueError` instead of returning
    an ambiguous value if the index is negative;
  * `map`, `zip`, `range`, `input`, `filter` work the same way (Py3k one)
    on both Python 2 and 3 (see #523 and #331).
Move the documentation for `zipwith` to the correct place.
Merge branch 'master' into pr/524