Provides generic `Atomic<T>` type

[taiki-e]

Requirements:

• Should completely avoid problems with padding. (crossbeam's AtomicCell and atomic-rs have problems with padding).
• Should be able to support structs and enums with multiple fields. (atomig doesn't support multiple fields)
• Should be forbid(unsafe_code) compatible on the user side. (I have mentioned in the past the approach of using unsafe marker trait.)
• Atomic<&T> must be sound (i.e., Relaxed ordering is not allowed), but Relaxed ordering should be allowed on plain data types like integers.

[taiki-e]

prototype: https://github.com/taiki-e/portable-atomic/compare/main...generic