Add a sleep function to the Core

[newpavlov]

It seems like a pretty fundamental function, which is currently missing from the Core. It could be as simple as sleep(uint64_t nanos).

[sunfishcode]

I'm the current API, the way to implement sleep is to use poll_oneoff, polling for a single __WASI_EVENTTYPE_CLOCK event.

[newpavlov]

Ah, good to know. :)