Increase MSRV to 1.63

[jswrenn]

This will allow us to progress on const-generics Require Rust 1.51 or newer , particularly #560. This entails:

• updating
  

  itertools/.github/workflows/ci.yml

  Line 47 in a0225ce

  toolchain: 1.43.1

• updating
  

  itertools/Cargo.toml

  Line 19 in a0225ce

  rust-version = "1.43.1"

• updating
  

  itertools/src/lib.rs

  Line 49 in a0225ce

  //! This version of itertools requires Rust 1.43.1 or later.

• auditing for allow'd clippy lints that can now be fixed
• auditing TODO comments that can now be fixed

[Philippe-Cholet]

And update

itertools/src/lib.rs

Line 49 in a0225ce

//! This version of itertools requires Rust 1.43.1 or later.

[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

itertools/src/concat_impl.rs

Lines 22 to 29 in a0225ce

	#[allow(deprecated)] //TODO: once msrv hits 1.51. replace `fold1` with `reduce`
	iterable
	.into_iter()
	.fold1(|mut a, b| {
	a.extend(b);
	a
	})
	.unwrap_or_default()

[jswrenn]

Yes, please do!