More CI checks

[Jules-Bertholet]

Splits out "unrelated" changes from #92 as requested.

• Check formatting
• Check clippy
• Check MSRV
• Check docs

[Manishearth]

This is still a lot of changes all in one commit.

Please split it up further, it can be in one PR but it should be reviewably split into multiple commits, that have descriptive commit messages when they're making nontrivial changes.

Where are all the extra tests coming from? What's with all of the changes to the python script? Some seem to be python formatting (fine), others actually change the structure/behavior of the code?

[Jules-Bertholet]

The changes to the Python script are to ensure that the Rust code files it outputs pass rustfmt. I didn't make any changes to how the normalization tests are generated; I just regenerated the file, which was outdated before (as not checked by CI).

[Jules-Bertholet]

I've split the PR into four commits.

[Manishearth]

@Jules-Bertholet okay, can you remove the Python changes? Let's have #[rustfmt::skip] on the generated file for now. Perhaps save those changes for a separate PR, I'd like to have a closer look at that and I don't want to block this on that.

[Jules-Bertholet]

can you remove the Python changes?

Done

[Manishearth]

CI fails

error: the item `FnMut` is imported redundantly
  --> src/normalize.rs:17:18
   |
17 | use core::{char, ops::FnMut};
   |                  ^^^^^^^^^^
  --> /rustc/c475e[23](https://github.com/unicode-rs/unicode-normalization/actions/runs/8099597628/job/22136519907?pr=93#step:4:24)03b551d726307c646181e0677af1e0069/library/std/src/prelude/mod.rs:115:13
   |
   = note: the item `FnMut` is already defined here
   |
   = note: `-D unused-imports` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unused_imports)]`