Introduce pgproto3 package

pgproto3 will wrap the message encoding and decoding for the PostgreSQL
frontend/backend protocol version 3.

notice_response.go

@@ -0,0 +1,13 @@
+package pgproto3
+
+type NoticeResponse ErrorResponse
+
+func (*NoticeResponse) Backend() {}
+
+func (dst *NoticeResponse) UnmarshalBinary(src []byte) error {
+	return (*ErrorResponse)(dst).UnmarshalBinary(src)
+}
+
+func (src *NoticeResponse) MarshalBinary() ([]byte, error) {
+	return (*ErrorResponse)(src).marshalBinary('N')
+}