Create Rc/Arc directly from mut slice

[eduardosm]

Right now, we have

impl From<&CStr> for Rc<CStr>
impl From<&OsStr> for Rc<OsStr>
impl From<&Path> for Rc<Path>
impl From<&str> for Rc<str>

and their Arc counterparts.

Even though a mut slice can be coerced into a non-mut one, I think it would be useful to have versions that can directly take &mut _.

For example, to be able to do something like:

let s: Rc<str> = chr.encode_utf8(&mut [0; 4]).into(); // the trait bound `Rc<str>: From<&mut str>` is not satisfied

[jieyouxu]

This will need a T-libs-api API Change Proposal (ACP).

[eduardosm]

Implemented in #129329.