Feature: Detect "exit" immediately if last process pipe is closed
(#58 by @clue)

This introduces a simple check to see if the program is already known to be
closed when the last process pipe is closed instead of relying on a periodic
timer. This simple change improves "exit" detection significantly for most
programs and does not cause a noticeable penalty for more advanced use cases.

Fix forward compatibility with upcoming EventLoop releases
(#56 by @clue)