monotonic clock: introduce duration type, split subscribe

[pchickey]

Implement the interface change submitted upstream as WebAssembly/wasi-clocks#53

We are introducing a duration type because it has a distinct meaning
from instant: an instant can only be compared to other instants
from the exact same monotonic-clock, whereas a duration represents
a duration of time which can be compared to any other duration of time.

The duration type is motivated, in part, by a desire to reuse it to
specify durations such as timeouts in other WASI proposals.

Instead of taking a boolean specifying whether the u64 is an absolute or
relative time, subscribe-instant takes an instant type and
subscribe-duration takes a duration type.