Increase MSRV to 1.63

[jswrenn]

This will allow us to progress on https://github.com/rust-itertools/itertools/labels/const-generics, particularly #560. This entails:

• updating https://github.com/rust-itertools/itertools/blob/a0225ceee50cafe9e974699e1cbf7fa9a1dcf901/.github/workflows/ci.yml#L47
• updating https://github.com/rust-itertools/itertools/blob/a0225ceee50cafe9e974699e1cbf7fa9a1dcf901/Cargo.toml#L19
• updating https://github.com/rust-itertools/itertools/blob/a0225ceee50cafe9e974699e1cbf7fa9a1dcf901/src/lib.rs#L49
• auditing for allow'd clippy lints that can now be fixed
• auditing TODO comments that can now be fixed

[Philippe-Cholet]

And update https://github.com/rust-itertools/itertools/blob/a0225ceee50cafe9e974699e1cbf7fa9a1dcf901/src/lib.rs#L49

[scottmcm]

👍 to 1.63 here. The more things can be from_fn instead of writing unsafe code here the better.

And anyone using 1.69 or greater -- probably nearly everyone, TBH -- will get a version with a bunch of internal optimizations to how map and from_fn work.

[Philippe-Cholet]

@kinto-b That's something you might want to do. The "big" part here is to grep clippy and see which lint(s) can be resolved, if any.

[kinto-b]

@Philippe-Cholet Sure thing :) I assume you mean, go through and see which #[allow(clippy::*)] can be dropped?

Shall I also action the TODOs such as https://github.com/rust-itertools/itertools/blob/a0225ceee50cafe9e974699e1cbf7fa9a1dcf901/src/concat_impl.rs#L22-L29

[jswrenn]

Yes, please do!