Previous commits went back and forth a bit on whether the various APIs
should use bytes or strings, but bytes appears to be a better answer,
because actual data in streams will always be 7-bit ASCII or raw
binary. There's no reason to apply the performance penalty of
constantly converting between bytes and strings.
One drawback now is that lots of code now has to have "b" prefixes on
strings, especially in tests, which inflates this commit quite a bit.