zstd compression for pg_dump

I found that our largest tables are 40% smaller and 20% faster to pipe
pg_dump -Fc -Z0 |zstd relative to native zlib

So I wondered how much better when integrated in pg_dump, and found that
there's some additional improvement, but a big disadvantage of piping through
zstd is that it's not identified as a PGDMP file, and, /usr/bin/file on centos7
fails to even identify zstd by its magic number..

I looked for previous discussion about alternate compressions, but didn't find
anything for pg_dump.

-- 
Justin