This switches from using curtsies-created paste events to creating paste events in bpython whenever 10 or more keypress events are queued up.

I think long term this is a good move because it's one less Curtsies-specific piece of input functionality, and I'd like to remove this feature from Curtsies and use Blessed for input code eventually because I trust @jquast to get it right more than I do myself. It's also better because now it helps the UI catch up any time it gets behind on events, e.g. from a fast typist on a slow machine, whereas before the bytes had to be read in the same system call to be considered a paste event. The paste threshold of 10 should be tuned for this, it should probably be much lower (2 or 3?).

If you don't like the combined_events coroutine I'd be happy to reimplement it as a class, I think that might be clearer but because the .send and next() interface was all we needed it seemed easy to write that way.