sum-of-multiples: Move to functional solution, add tests

[sjakobi]

Closes #139, #147.

[sjakobi]

@Dog, @mmakaay: Because your PRs had merge conflicts they're now included in this PR. Do you think that it's a sensible package as a whole?

[Dog]

My initial concern is I think the example assumes zero is the first multiple in the list. I'll try to play with it later tonight.

[sjakobi]

My initial concern is I think the example assumes zero is the first multiple in the list.

That should be ok under the assumptions given in the test file (the input factors are non-negative, unique and sorted in ascending order).

[Dog]

Then it looks good to me.

[kytrinyx]

Thanks, everyone!