Auto merge of #56988 - alexcrichton:monotonic-instant, r=sfackler

std: Force `Instant::now()` to be monotonic

This commit is an attempt to force `Instant::now` to be monotonic
through any means possible. We tried relying on OS/hardware/clock
implementations, but those seem buggy enough that we can't rely on them
in practice. This commit implements the same hammer Firefox recently
implemented (noted in #56612) which is to just keep whatever the lastest
`Instant::now()` return value was in memory, returning that instead of
the OS looks like it's moving backwards.

Closes #48514
Closes #49281
cc #51648
cc #56560
Closes #56612
Closes #56940