Bump to 2021 edition & inline formats

[nyurik]

• Bump Rust edition 2018 -> 2021
• Use new inline formatting, i.e. format!("{v}") instead of format!("{}", v) (simple cases only)

[djc]

Hey, thanks for this. This is a bit of a grab bag of changes. Are you gearing up for further contributions here, what's the angle?

Mostly it looks good. I think I'd like to stick with 2018 edition for now so we can keep a lower minimum supported Rust version for now. I don't think we test that explicitly in CI yet, maybe you could add that, and see what's the oldest version that currently works?

[nyurik]

hi @djc , mostly this was the result of me looking through the code - I try to do minor things when i go over it, this way i feel i get a bit better understanding of it :)

The 2018 edition and the inline vars in the format str are connected -- the format was introduced in 1.58, after the 2021 edition (1.56), so these kinda have to go together.

If you think MSRV should be less than 1.56 (should be documented somewhere i guess), i could split this into two PRs

[djc]

Yes, we should stick to less than 1.56, would be nice if you can set that in CI and add a note to the README.

[nyurik]

I moved the simpler ones to a separate PR. CI changes are still a TODO

[djc]

Since we're on hashbrown 0.12 now which requires Rust 1.56 anyway, going to merge this. Thanks!