Create aliases for iters.head (first) and iters.tail (rest)

`iters.first` and `iters.rest` as aliases for `iters.head` and `iters.tail` respectively, as used in programming languages like Clojure.

`first` is already in use inside `iters.iter_except`, but the function is already complete and tested, hence such conflict wouldn't cause any harm in my opinion. Optionally, a trailing `_` can be added to make it obvious, provided that it conforms to PEP 8 (http://www.python.org/dev/peps/pep-0008/). For example,

```
if first_ is not None:
    ...
```