Add test that compares docstrings of functions in `toolz` and `cytoolz`.

This is to improve maintainability.  If docstrings don't match, then a convenient diff is printed.  Docstrings in `cytoolz` may have additional lines at the beginning and end.

This PR is a first-stab implementation, and is probably good enough.  Maintainability of `cytoolz` is very important, though, because we want to keep up with the development of `toolz` with the minimum amount of effort.  If this PR can be improved or if there is a better way to do this, I would like to hear about it.