Fix UdpCodec::encode (#85)

*     Refactor UDP SendDgram & RecvDgram

    Get rid of unnamed structs in the favor of private structs with named fields

*     Change the signature of UdpCodec::encode

    Now it is:

    ```
        fn encode(&mut self, msg: Self::Out, buf: &mut Vec<u8>) -> Result<SocketAddr, Self::Error>;
    ```

    Closes https://github.com/tokio-rs/tokio/issues/79

* Fix compilation error from `mio` crate

examples/connect.rs

@@ -219,14 +219,15 @@ mod udp {
     impl UdpCodec for Bytes {
         type In = (SocketAddr, Vec<u8>);
         type Out = (SocketAddr, Vec<u8>);
+        type Error = io::Error;
 
         fn decode(&mut self, addr: &SocketAddr, buf: &[u8]) -> io::Result<Self::In> {
             Ok((*addr, buf.to_vec()))
         }
 
-        fn encode(&mut self, (addr, buf): Self::Out, into: &mut Vec<u8>) -> SocketAddr {
+        fn encode(&mut self, (addr, buf): Self::Out, into: &mut Vec<u8>) -> io::Result<SocketAddr> {
             into.extend(buf);
-            addr
+            Ok(addr)
         }
     }
 }

examples/udp-codec.rs

@@ -24,14 +24,15 @@ pub struct LineCodec;
 impl UdpCodec for LineCodec {
     type In = (SocketAddr, Vec<u8>);
     type Out = (SocketAddr, Vec<u8>);
+    type Error = io::Error;
 
     fn decode(&mut self, addr: &SocketAddr, buf: &[u8]) -> io::Result<Self::In> {
         Ok((*addr, buf.to_vec()))
     }
 
-    fn encode(&mut self, (addr, buf): Self::Out, into: &mut Vec<u8>) -> SocketAddr {
+    fn encode(&mut self, (addr, buf): Self::Out, into: &mut Vec<u8>) -> io::Result<SocketAddr> {
         into.extend(buf);
-        addr
+        Ok(addr)
     }
 }