std: implement sleep_until for Fuchsia

[joboet]

Tracking issue: #113752

Unfortunately Fuchsia's clock_nanosleep is only a stub. The kernel's zx_nanosleep syscall however does exactly what's needed.

I've done a few drive-by changes in the two outer commits, our uses of zx_time_t use the zx_instant_mono_t alias nowadays and the syscall table in the sleep_until documentation didn't mark the function names as code.

@rustbot ping fuchsia

[rustbot]

Hey friends of Fuchsia! This issue could use some guidance on how this should be
resolved/implemented on Fuchsia. Could one of you weigh in?

cc @erickt @Nashenas88

[rustbot]

r? @ibraheemdev

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[Nashenas88]

Sorry for the delay here. I was on vacation, then out sick for a bit.

The implementation looks correct to me. Thank you for updating the zx_time_ts to zx_instant_mono_ts! I'll take this back to the team since I believe we should have updated the rust bindings when that change was made.

One nit on the commit comment: while most usages of zx_time_t are now zx_instant_mono_t, there are still usages where zx_time_t is valid and distinct from zx_instant_mono_t. I believe this is always the case where the exact timeline used (e.g. boot or monotonic) is configurable. For example, with zx_timer_set.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.