Rollup merge of #130046 - RalfJung:const_str_as_mut, r=dtolnay

str: make as_mut_ptr and as_bytes_mut unstably const

`@rust-lang/libs-api` the corresponding non-mutable methods are already const fn, so this seems pretty trivial. I hope this is small enough that it does not need an ACP? :)

I would like to get these stabilized ASAP because I want to avoid people doing `s.as_ptr().cast_mut()`, which is UB if they ever write to it, but is already const-stable.

TODO: create a tracking issue.

Author: matthiaskrgr