Instant and Duration subscriptions for crux_time

[StuartHarris]

Updates crux_time capability to be more closely aligned with wasi-clocks.

• update now to return an Instant, which represents a UTC point in time (number of seconds since the epoch and number of nanoseconds since the last second).
• add subscribe_duration and its async counterpart. The shell side of the capability responds when the requested duration has elapsed.
• add subscribe_instant and its async counterpart. The shell side of the capability responds when the requested instant has arrived.
• Add a test that demonstrates a debounce based on subscribe_duration

[wasnotrice]

@StuartHarris great update to time!

[obmarg]

Nice, I just have a few naming nitpicks

[obmarg]

Though I just skimmed that spec you linked, and have realised that's probably where you got the names. Never mind then