Decide on a merged import style

[lambda-fairy]

See https://rust-lang.github.io/rustfmt/#imports_granularity

Currently, Maud uses the "module" style. But we should consider switching to "crate" style instead.

[lambda-fairy]

Only disadvantage I can see is that imports_granularity is still unstable (rust-lang/rustfmt#4681). So we'll need everyone who contributes to Maud to use nightly.

I don't think that's an issue, though, since the tests are already nightly-only.