Summary:
This diff implements `coro_wait`, an annotated helper to block on the execution of a coroutine.

The goal here is to provide an easy way to migrate from fibers to coroutines.

Reviewed By: A5he

Differential Revision: D21978610

fbshipit-source-id: d59a222978d9db2963656312e71c7dcb2b9a11e9