Auto merge of #96078 - udoprog:refcounted-str-to-u8, r=dtolnay

Implement str to [u8] conversion for refcounted containers

This seems motivated to complete the APIs for shared containers since we already have similar allocation-free conversions for strings like `From<Box<[u8]>> for Box<str>`.

Insta-stable since it's a new trait impl?

diff --cc library/alloc/src/rc.rs
Simple merge