Add a polling interface for monotonic clocks.

[sunfishcode]

The new wasi-poll API has a pollable type. Add subscribe methods to the monotonic clock types to allow polling for timeouts.

This adds a dependency on wasi-io, and is currently implemented in the repository in a low-tech way, which allows the generated documentation to work, though it's likely to evolve.

[sunfishcode]

As before, merging this so that it's easier to see in the main repo. Comments and questions welcome!