Refactor: Use builtin `enumerate(...)` rather than `zip(range, ...)`

I had added a few instances of enumerating elements in a list for iteration
with:

    for n, x in izip(range(len(l), l):
        ...

This is a much less clear than using the `enumerate` builtin and requires
importing `izip`.  I had used `izip` and `range` because I didn't know
about `enumerate` at the time.