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, zx_time_t is now called zx_instant_mono_t and the syscall table in the sleep_until documentation contained a few unnecessary square brackets (and wasn't that nice to look at).

@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

rustbot has assigned @ibraheemdev.
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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 8 candidates
• Random selection from Mark-Simulacrum, ibraheemdev

[rust-bors]

☔ The latest upstream changes (presumably #153377) made this pull request unmergeable. Please resolve the merge conflicts.