Implement From<{Rc,Arc}<str>> for [u8] counterpart

[nvzqz]

This enables safely converting from Arc<str> or Rc<str> to Arc<[u8]> or Rc<[u8]> respectively.

Currently the following steps are needed outside of the standard library:

fn from_str(s: Arc<str>) -> Arc<[u8]> {
    let len: usize = s.len();
    let ptr: *const u8 = Arc::into_raw(s).cast();
    let raw: *const [u8] = std::ptr::slice_from_raw_parts(ptr, len);
    unsafe { Arc::from_raw(raw) }
}

[rust-highfive]

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[rust-highfive]

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking alloc v0.0.0 (/checkout/library/alloc)
    Checking cfg-if v0.1.10
    Checking adler v0.2.3
    Checking rustc-demangle v0.1.21
error[E0119]: conflicting implementations of trait `core::convert::From<rc::Rc<str>>` for type `rc::Rc<[u8]>`
     |
     |
1900 | impl From<Rc<str>> for Rc<[u8]> {
     | ------------------------------- first implementation here
...
1983 | impl From<Rc<str>> for Rc<[u8]> {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `rc::Rc<[u8]>`

error[E0119]: conflicting implementations of trait `core::convert::From<sync::Arc<str>>` for type `sync::Arc<[u8]>`
     |
     |
2501 | impl From<Arc<str>> for Arc<[u8]> {
     | --------------------------------- first implementation here
...
2584 | impl From<Arc<str>> for Arc<[u8]> {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `sync::Arc<[u8]>`
error[E0711]: feature `shared_from_str` is declared stable since 1.62.0, but was previously declared stable since 1.63.0
    --> library/alloc/src/rc.rs:1982:1
     |
     |
1982 | #[stable(feature = "shared_from_str", since = "1.62.0")]

error[E0711]: feature `shared_from_str` is declared stable since 1.62.0, but was previously declared stable since 1.63.0
    --> library/alloc/src/sync.rs:2583:1
     |
     |
2583 | #[stable(feature = "shared_from_str", since = "1.62.0")]

For more information about this error, try `rustc --explain E0119`.
error: could not compile `alloc` due to 4 previous errors
Build completed unsuccessfully in 0:01:39

[nvzqz]

Apparently this was already done in #96078! Thanks @udoprog!