options:
- do nothing, document user must depend on `portable-atomic` and enable either `critical-section` or `unsafe-assume-single-core`
- reexport portable-atomic features `critical-section` and `unsafe-assume-single-core`, document user must enable one of them
- feature-gate AtomicDevice with a default-off feature. breaking change, and document.
- feature-gate AtomicDevice with a default-on feature, and document.