`stimes` should be defined on `0` for all monoids

I assume that `stimes 0 xs` should be `mempty` for all monoids.

However, this is something that is easy to miss + many of the implementations out there are "broken" in that regard.

Should we test for this in `monoidLaws`?

Start by locating `monoidLaws` and the existing laws around `stimes` in the repository. Check how laws are expressed and tested, then determine whether `stimes 0 xs == mempty` can be added as a general monoid law; done means the law is included and the relevant test suite passes.

Written by the indexing model from the issue text.