paste: missing support for `-z` flag

[jtracey]

from GNU's paste -h:

-z, --zero-terminated    line delimiter is NUL, not newline

Similar to quite a few other utils, e.g. #2351 and #2880. Most of the remaining failing GNU paste tests are blocked on this.

As was the case with join, it's probably a good idea to convert paste to operating on bytes instead of strings first.