Use more recent Rust features and bump MSRV

[hniksic]

This PR was inspired by the stabilization of std::sync::OnceLock, which allows this crate to omit the dependency to the once_cell external crate. This seems like a benefit that would justify an MSRV bump. And if we're bumping MSRV anyway, then we can also use usize::BITS and simplify the example using scoped threads.

[hniksic]

I've also removed the unreachable module because Rust 1.58 introduced Option::unwrap_unchecked(), Result::unwrap_unchecked(), and Result::unwrap_err_unchecked(). Only Option::unwrap_none_unchecked() is missing, but it was not used by the crate anyway.

If this is undesirable, I'll revert or remove that commit. (The same of course applies to other changes grouped in this PR, which are all in separate commits, and therefore easy to undo.)