Don't redraw the status line every time when a batch of output occurs at once

[jwodder]

If the child process emits a large number of output lines at once, elapsed will currently clear & redraw the status line before & after each & every one. Better handling of such situations is needed.

• Possible solution: Split the select-loop into two tasks in a multi-threaded runtime. Task one uses a select-loop to receive ticks & subprocess events, and then it sends them through a channel to task two. Task two calls Receiver::recv().await in a loop, and each time this future completes, it clears the status line, processes the event, and then loops over Receiver::try_recv() — processing any more events received in quick succession — until it returns Empty, and only after that is the status line redrawn.