Mark Arc function get_mut and method make_unique unsafe

Mark Arc function get_mut and method make_unique unsafe

This is a temporary mitigation for issue #24880 which points out that
these functions are racy in a particular situation where weak pointers
exist.

To mitigate this, mark the functions unsafe until this can be fixed or
another decision is made.

This is a breaking change to unstable API, because the new version
requires an `unsafe` block. Review carefully if weak pointers may race
for any uses of this API and consider abandoning it.

[breaking-change]