[$] READ_ONCE(), WRITE_ONCE(), but not for Rust

The READ_ONCE() and WRITE_ONCE() macros are heavily used within the kernel; there are nearly 8,000 call sites for READ_ONCE(). They are key to the implementation of many lockless algorithms and can be necessary for some types of device-memory access. So one might think that, as the amount of Rust code in the kernel increases, there would be a place for Rust versions of these macros as well. The truth of the matter, though, is that the Rust community seems to want to take a different approach to concurrent data access.