API - Support Stream+Sink for ProxySocket?

[ibigbug]

The tcp stream does take opaque AsyncRead and AsyncWrite as underlying io https://docs.rs/shadowsocks/latest/shadowsocks/relay/tcprelay/proxy_stream/client/struct.ProxyClientStream.html#impl-AsyncRead-for-ProxyClientStream%3CS%3E

thoughts about having the udp socket to take Stream + Sink too ?

https://docs.rs/shadowsocks/latest/shadowsocks/relay/udprelay/proxy_socket/struct.ProxySocket.html

so it can be applied to proxy more general udp transports