Use Once from std when available

[Kixunil]

The current code in secp256k1_context_preallocated_create uses hand-rolled busy loop even when Once is available potentially wasting CPU. Abstracting them away and conditionally using Once from std would avoid the waste.

[elichai]

I think this is only for the fuzzing configuration?

[Kixunil]

Yeah, didn't notice, probably not worth the effort.

[TheBlueMatt]

Right, the point of the hand-rolled code there is precisely to avoid relying on std/thread sleeping in fuzzing. Honestly I forget what the configuration I was trying to match was when I wrote it, proabbly driller or similar.