Return values

[chriso]

This PR adds optional support for coroutine return values.

If the coroutine was constructed with a new coroutine.NewWithReturn(func () R) constructor, the return value can be retrieved using:

for c.Next() {
   // handle yields
}
var result R = c.Result()

The reason we haven't added support for passing arguments to coroutines is because it's easy to pass in data using a closure. On the other hand, getting data out of coroutines has so far been problematic. Although you can create a final yield point to pass return values out of the coroutine, the issue in #84 arises.

This PR provides a native way to get data out of a coroutine, so that you don't have to manually create a final yield point.

To keep things simple, I chose to reuse the yield type R rather than introduce a new generic type parameter for the return value.

[achille-roussel]

🚢