Request, create empty promise object that can be set later outside of a coroutine.

Hi,

I was wondering if it would be possible to add a method that would set a coroutine object from the "outside".
For example
```
boost::cobalt::promise request;
// save the promise somewhere in a data struct
// ....
// recieve some network data
// match it with a request
request.set(data);
```
this would then resume any coroutine that is currently co_awaited on this promise. I found a way around this by using async channels but was wondering if it would be at all possible to do this without them?