Use protobuf’s Gzip{Input,Output}Stream wrapper around zlib

This removes our direct zlib dependency (although of course protobuf
still uses it internally), removes a fixed 4 MiB buffer and its
corresponding limit on the terminal size, reduces some string copying,
and deletes some code.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

(Closes #230.)