In response to#55

Two new methods inUI:

1. async_context creates a token (iui::concurrent::Context) which can serve as evidence thatUI::init() has run successfully; this token isSend
2. spawn schedules an async function or future on the main thread. Since it's a method inUI it can only be run in the main thread. It returns control to the main loop when it blocks and queues itself again when it's ready again.

Two methods inconcurrent::Context

1. queue_main behaves likeUI::queue_main in that it schedules a closure or function to run on the main thread. UnlikeUI::queue_main it can be run in other threads but requires the closure to beSend.
2. spawn behaves likeUI::spawn but can be run from other threads; requires the future to beSend

UsingUI::spawn and an async channel such as provided bytokio::sync::mpsc we can wait for things to happen on other threads without busy waiting or blocking other events and then update our widgets accordingly.