I'd like to have `group_by` in a separate file from `map`.

Ideally each enumerable method will have their own file, and that file with have two suites in it:

1) The problem, solved with `each`
2) The problem, solved with whatever method we're focusing on

If I can get my act together, the two suites will have the exact same tests in them, same data, same assertions.

What do you think?