Add built-in support for handling output from an async enumerator

[jhickson]

Possible interface

public interface IActor
{
    /* ... */

   Task Enumerate<T>(IAsyncEnumerator<T> enumerator, Action<T> currentValueHandler, Action completionHandler = null);

   Task Enumerate<T>(IAsyncEnumerator<T> enumerator, Func<T, Task> currentValueAsyncHandler, Action completionHandler = null);
}

Usage

IAsyncEnumerator<int> enumerator = /* .... */;

_actor.Enumerate(enumerator, current => {}, () => {});

Notes

• Need to handle case where completionHandler is actually marked as async - i.e. it's really of type Func<Task>.
• Is Enumerate the best name?