Add `consume` to efficiently consume an iterator

The easiest way to do this is to add `cytoolz` as a dependency, which has a _very_ efficient `consume`.

When I first developed `benchtoolz`, I wanted to use `toolz`, but I didn't know what magic would be needed and whether using a package that I intend to test would be a good idea.  Adding `cytoolz` will give us other useful functions to use too :)
