Consider providing fallback implementation that uses lock per atomic type

[taiki-e]

The current lock-based fallback uses global locking for memory layout compatibility with standard library atomic types. This is the approach also used in C++ std::atomic, atomic-rs, and crossbeam AtomicCell, which is not so bad, but not best from a point of view of performance.

portable-atomic/src/imp/fallback/mod.rs

Lines 1 to 10 in c0a175e

	// Fallback implementation using global locks.
	//
	// This implementation uses seqlock for global locks.
	//
	// This is basically based on global locks in crossbeam-utils's `AtomicCell`,
	// but seqlock is implemented in a way that does not depend on UB
	// (see comments in optimistic_read method in atomic! macro for details).
	//
	// Note that we cannot use a lock per atomic type, since the in-memory representation of the atomic
	// type and the value type must be the same.

It may be possible to provide something like this as a parameter or from another module.

[taiki-e]

Alternatively, we might want to allow custom fallback implementation.