Add GitHub CI and fix all found issues

[tisonkun]

@seanmonstar

First I'd like to add a GitHub CI coverage for this repo, and then I go through all the files for fmt and clippy.

Let me know if it's desired or if anything I can help to move this crate to 1.0.

[seanmonstar]

Thanks for offering to help! To see what's currently blocking this library, take a look at the pinned #153.

[tisonkun]

@seanmonstar Thanks for pointing out. I'm spending some time on the issue now.

What do you think of this PR? Generally, I'd start with a CI-covered workspace to help make progress more confidently.