Let's move off our custom QuaZip. In the olden times we needed the
custom version of QuaZip, as it was basically unmaintained and on
SourceForge (eww). But nowadays it's maintained and on GitHub. See
new GitHub page: https://github.com/stachenov/quazip
`a << b` is undefined when `a` is negative, and `a >> b` is
implementation-defined. The correct thing to do here is to cast to
unsigned, swap the bytes there and then swap back.
This also improves performance on some compilers: Clang is smart enough
to recognise that we're byteswapping here and reduce it to a single
`bswap` instruction on x86_64, but only for the unsigned versions.
