Minor (potential) improvements

[Rolv-Apneseth]

Feel free to decide if this is even necessary but these are the changes in this small PR:

• Format using rustfmt
• Fix clippy warnings
• Put all tests in separate modules

I suppose the main thing is the tests going into modules, as that cleans up the test specific imports and puts all tests behind a #[cfg(test)] attribute which should keep all those test functions from needing to be included in the final compilation even when not testing. This seemed to lead to saving an admittedly tiny 2k (48k to 46k) from the libshlex.rlib with cargo build --lib --release on my machine.

[fenhl]

Please split these changes into a separate PR for each bullet point so they can be reviewed individually.

[Rolv-Apneseth]

Sure yeah. Would it be alright to keep the formatting and linting stuff together, so it's 2 separate PRs?

[fenhl]

I'd prefer 3 PRs since it's the formatting stuff that I'm least inclined to accept currently.

[Rolv-Apneseth]

Alright no problem. I'll close this for now and get to it hopefully later tonight.