Fix up issues with map usage.
In python 3, the map function has changed a little bit. In particular, it now

stops iteration when the first iterator it encounters is exhausted. This means

in cases where we were applying the identity function (using None as the

first argument), it would have different behavior. I've changed it to use

itertools.izip_longest (which in python 3 will just be called zip_longest).
There were also a couple places where the 2to3 tool suggested we replace the

map with a for loop. I've done so, since it seemed to make sense.

Ran unit tests.