rustfmt: Set imports_granularity to Module

We do not have a consistent style that we use for import statements, which is a bit of a pain because that is nothing one should have to think about.

Let cargo fmt do the job for us by saying so in rustfmt.toml. The “Module” granularity seems to fit our current “style” most closely, so let’s use that one.

(As suggested by Marc-André on MR !41 (merged) and issue #16 (closed), but using the “Module” granularity just because that causes less changes than we’d get with “Crate”.)