Add functions to allocate zeroed Arc and Rc

[a1phyr]

These require rustc 1.82

[zachs18]

so that this will still compile on targets without Arc

[zachs18]

Also: IIUC it is currently up in the air if it is valid to write to the pointer returned from {Arc,Rc}::as_ptr: rust-lang/rust#87862 rust-lang/rust#104337 , so ideally these would use {Arc,Rc}::get_mut(&mut arc).unwrap() at least until that is clarified¹.

Footnotes

1. The compiler can optimize out the unwrap panic branch for Rc but not Arc, but it should still never be possible to panic at runtime since the Arc is newly allocated ↩

[a1phyr]

As you wish. I updated the code to use write_zeroes and fill_zeroes, it is cleaner like that if we use get_mut

[Lokathor]

released in bytemuck-1.20