Close pipes when dropping them, check for closed pipe when flushing a writer

[maxbrunsfeld]

Thanks for sharing this library! This PR makes two semi-orthogonal improvements:

1. When attempting to flush a closed PipeWriter, return an error immediately. This makes flush consistent with write, and it avoids an indefinite hang that you would otherwise get when attempting to flush a closed pipe.
2. Mark a pipe as closed when either the PipeWriter or PipeReader are dropped. Previously, when one end of a pipe was dropped, any read/writes to the other end would hang indefinitely, which does not seem like it would ever be a useful behavior.