Replace pqxx::bytes.

[jtv]

As the documentation says, the recently introduced type aliases pqxx::bytes and pqxx::bytes_view need to be changed. They are currently aliases for std::basic_string<std::byte> and std::basic_string_view<std::byte> respectively. I've already changed bytes_view, but bytes is harder.

Unfortunately this will break code. There are substantial differences in constructor signatures between std::basic_string and std::vector``. I've tried working around that by deriving my own class from std::vector`, but besides being a surprising amount of work, it negates the whole idea of using a standard library type!