Sweetasync

- remove the Bundle class since you can instead pass the same channel
object around and get the same results, or use an EventEmitter.
- add backpressure on channels for the number of coroutines awaiting
a message or awaiting to send a message.
- fix `__all__` exports: make `asyncify` accessible from the top-level namespace,
as per the documentation, and remove `awaitable` (old name for `as_await`).

- interface sugar for sending messages with channels: it isn't necessary
anymore to put multiple messages into a list or a tuple to send them
simultaneously. __call__ and send accept any number of argument.
When no argument is given, `None` is sent.
- improved documentation
- methods to close channels are now idempotent