Add generic Atomic<T> type

[taiki-e]

Note: this patch currently adds generic atomic type to the main portable-atomic crate, but should be to the portable-atomic-util crate.

• Support for various operations on Copy types and support for swap and store on non-Copy types.
• Support for primitives, immutable references, function pointers, NonNull, NoneZero, etc.
• Support for user-defined structs and enums, including those with multiple fields and padding. (via #[derive(Atomicable)])
• Support for user-defined unions. (via #[derive(Atomicable)])

This should meet the requirements that I said in #1.

Requirements:

• Should completely avoid problems with padding. (Soundness of AtomicCell::compare_exchange is dubious crossbeam-rs/crossbeam#315).
• 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.

Closes #1