Fix buffered.Reader.append to accept any ByteSeq. (#1644)
Fix buffered.Reader.append to accept any ByteSeq. (#1644)

This is an issue of principle-of-least-surprise, since buffered.Writer
emits ByteSeqs in its output, so having buffered.Reader accept ByteSeqs
at its input makes it symmetrical and least surprising.

This doesn't break any user code, since it only makes an argument type
more general, and does not make any return types more general.
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