Other handles can only be used within the loop itself, but Async is intended
to be used from another thread. This introduces the possibility of a race
condition between the loop being destroyed and the Async being destroyed.
Change Async to keep a weak reference to a loop and check it before performing
libuv operations.
This allows the called function to pass along the promise to another
asynchronous callback.
To avoid memory allocations, add a home-rolled, simplified, non-allocating
version of std::promise and std::future as wpi::promise and wpi::future.
