inc, square, map, range, and list are all either from the standard library or can be written in this file; `groupby` isn't.  In general I avoid imports across branches in a project directory tree.  This is for a few reasons. 

1.  A future failure in `groupby` shouldn't cause an error inside functoolz.  This tests `groupby` in a nonlocal way.
2.  If we ever wanted to change the organization structure of these submodules (like we just did in #3) then these cross links will become a PITA