AsyncChannelKit
A simple implementation of an async channel for use with AsyncSequence and AsyncIterator.
When the next() function is called it is expected to return a value asynchronusly for an async for loop.
If the values are not created in response to calling this function these values will have to be provided
somehow. This channel supports sending values which the next() can return.
In the unit tests, a channel is created and given to the sequence. When the iterator is created it is
given the same channel. Calling send on the channel will make new values available. When the next
function is called it creates a continuation which will be matched to a value so it can be returned.
Calling terminate will use a continuation to send nil which tells the for loop it is done.
All of the work done by the channel is done as an actor type so that it is thread-safe.
