Add as_unsafe_cell() for Cell and RefCell

[kmcallister]

Fixes #18131.

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[alexcrichton]

This does unfortunately allow access to the public field of UnsafeCell to allow illegal access in otherwise safe code, so these may want to be unsafe for now unless we can figure out a better solution for the public field in the UnsafeCell.

Also, can you move the tests to src/libcoretest?

[kmcallister]

Addressed review feedback. I can squash this into a single commit once you're done reviewing.

[alexcrichton]

Ok, looks good to me. Could you also mark the functions as #[experimental] instead of #[unstable]? That's generally our default for new code added.

[kmcallister]

Could you also mark the functions as #[experimental] instead of #[unstable]?

Rebased and squashed with this change. Thanks for the review!