Add Fuchsia to actually_monotonic

[joshlf]

std's internal actually_monotonic function is used to determine which platforms' clocks are truly monotonic. Those that aren't (that have been observed to sometimes go backwards in practice) require more expensive machinery to ensure monotonicity of the Instant type on those platforms.

Fuchsia provides a truly monotonic clock, so we should add it to the list of platforms for which actually_monotonic returns true.

[joshlf]

cc @cramertj @tmandry