std: Make the generic atomics take unsafe pointers

These mutate values behind references that are Freeze, which is not
allowed.
std: Make the generic atomics in `sync::atomics` private

I'm not comfortable exposing public functions that purport to do
atomic operations on arbitrary T.
std: Remove AtomicU64

Support for this is less universal than for word-size things;
it has no users; i'd rather play it safe.