[Concurrency] Continuations for interfacing async tasks with synchronous code

[koher (Yuta Koshizawa)]

@Joe_Groff

The API is intentionally identical to the Unsafe variants, so that code
can switch easily between the checked and unchecked variants.

How about switching checked or unchecked automatically depending on if -Ounchecked (or maybe -O) flag is on? It seems consistent with precondition (or assert).