Updates repo metadata and formats the codes (part of #62)

[cxw620]

This PR adds rustfmt config and formats the codes, making it easier to merge later commits.

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Part of #62.

[djc]

This is much more reviewable, thanks!

Suggest getting rid of the "PR62: partial" prefixes on commit messages which are pretty useless and crowd out the actual content of the messages.

[djc]

Breaking changes: explicitly set rust-edition to 2021.

That should be in a separate commit.

[cxw620]

Breaking changes: explicitly set rust-edition to 2021.

That should be in a separate commit.

My mistake, Rust edition has been already set to 2021, I didn't change it.

[djc]

This is looking great, thanks for the iterations!

[cxw620]

For formatting, I think we can take rustls as an example, splitting stable and unstable config and add CI like this?

https://github.com/rustls/rustls/blob/a84d3e47388f0aa57ed96a81d4f85a46ffdab439/.github/workflows/build.yml#L429-L448

Shall this be done in a separate PR or here? I think we need to sort the import and modules.

[djc]

For formatting, I think we can take rustls as an example, splitting stable and unstable config and add CI like this?

https://github.com/rustls/rustls/blob/a84d3e47388f0aa57ed96a81d4f85a46ffdab439/.github/workflows/build.yml#L429-L448

This shall be done in a separate PR. I think we need to sort the import and modules.

Sounds okay to me. Given that this is a much smaller crate, I think it would also be fine to drop the unstable config + CI for now and just do an initial pass to use StdExternalCrate imports by hand.