Auto merge of #41096 - clarcharr:as_bytes_mut, r=alexcrichton

Auto merge of #41096 - clarcharr:as_bytes_mut, r=alexcrichton

Reduce str transmutes, add mut versions of methods.

When I was working on the various parts involved in #40380 one of the comments I got was the excess of transmutes necessary to make the changes work. This is part of a set of multiple changes I'd like to offer to fix this problem.

I think that having these methods is reasonable because they're already possible via transmutes, and it makes the code that uses them safer. I can also add `pub(crate)` to these methods for now if the libs team would rather not expose them to the public without an RFC.