Message-related functionality factored out into msg_t class.

This patch addresses serveral issues:
1. It gathers message related functionality scattered over whole
   codebase into a single class.
2. It makes zmq_msg_t an opaque datatype. Internals of the class
   don't pollute zmq.h header file.
3. zmq_msg_t size decreases from 48 to 32 bytes. That saves ~33%
   of memory in scenarios with large amount of small messages.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/decoder.hpp

@@ -27,10 +27,9 @@
 #include <algorithm>
 
 #include "err.hpp"
+#include "msg.hpp"
 #include "stdint.hpp"
 
-#include "../include/zmq.h"
-
 namespace zmq
 {
 
@@ -196,7 +195,7 @@ namespace zmq
 
         struct i_inout *destination;
         unsigned char tmpbuf [8];
-        ::zmq_msg_t in_progress;
+        msg_t in_progress;
 
         int64_t maxmsgsize;