Tweaks to support Edition 2024 of Rust in future

[kennykerr]

While the 2024 edition of Rust is out, switching isn't so easy since it also requires bumping the MSRV on all crates. Still, making what changes are possible early on does help with eventual transition and can catch problems and blockers early. So I started testing with 2024 and encountered a few notable thigs:

• extern blocks must be unsafe, but since this is not compatible with 2021 I have to leave that for now.
• no_mangle is considered an unsafe attribute, but that is also incompatible with 2021.
• Unsafe code within unsafe functions must be wrapped in unsafe blocks. Since this is compatible with 2021 I've gone ahead and applied this change.
• There are some lifetime changes that restrict lifetime promotion particularly for const values. I've gone ahead and fixed those. Notably the h! macro cannot return an HSTRING reference to a const value as Rust thinks the HSTRING is a temporary and will be dropped as the expression returns. Making it static solves this problem by explicitly promoting to static lifetime. This has the downside that the result of h! is no longer const but that's less of an issue than it not living in static memory which is the whole point.