sync/atomic: add Uint64Pair

I suggest CAS2 is implemented in the atomic package similar to atomic.align64, operational for hardware that supports it.

possibly also:

• atomic OR
• LL/SC Load-link/store-conditional

—

• With the 2022 memory model, go1.19 atomics became useful after years of unproductive near-sighted discussions
• However, wait-free algorithms now use CAS2, ie. 128-bit atomic, which is supported on most cpus but not in Go
• implementing CAS spinners in lock-free applications produces long thread wait-times or 100% cpu, so wait-free is better: threads are neither suspended or spinning. We will have 100 cores and 100 GiB RAM from Apple soon enough
• what is desired is a performant wait-free queue and wait-free free-list stack
• fast-path of wCQ Nikolaev Ravindran 2022 and wfq Yang Mellor-Crummey 2016
• this would enable an initial thread-pool to have wait-free and allocation-free data sink

darwin-arm64 darwin-amd64 linux-amd64

I think also atomic generics could do with another polish Like this AtomicMax that is more flexible in what underlying integer types and named types can be used: https://github.com/haraldrudell/parl/blob/main/atomic-max.go

• by inventing align64 only in atomic, 64-bit things must go in atomic