Use BINARY mode for results

[craff]

Hello,

I read in the documentation of postgresql that results of query can be sent to the client in BINARY, I guess it should
improve efficiency and could even be simpler (the coding and decoding with string are often catchy).

So it would be nice if pgx whas using BINARY when possible instead of STRING ?

[brendanlong]

The complicated thing with binary mode is that it's mostly undocumented ("do whatever libpq does"), so the code will probably be simpler once it's implemented, but it's much harder to implement. There's some discussion of it on #38 (comment)