Simplify various fixers

Subclass the corresponding fixer from lib2to3 to ease code maintenance by shifting work to python-dev. Covers filter, map, print, and zip.

Closes #73 and #66 .
Can I ask for a docstring here? As I understand the grammar, this would also return True for a list literal; that's irrelevant for what we want to use it for, because we only need to distinguish between function calls, listcomps and None, but it's worth a bit of explanation in the source.