Buffer IMAP client writes

async-imap does not do its own buffering, but calls flush() after
sending each command. Using BufWriter reduces the number of write()
system calls used to send a single command.

Note that BufWriter is set up on top of TLS streams, because
we can't guarantee that TLS libraries flush the stream before
waiting for response.

src/lib.rs

@@ -100,6 +100,7 @@ mod dehtml;
 mod authres;
 mod color;
 pub mod html;
+mod net;
 pub mod plaintext;
 mod ratelimit;
 pub mod summary;