msgpack: Fixes for 'Unpacker' and 'Unpacked'

Add a constructor to 'Unpacked' to call 'msgpack_unpacked_init'.

Let 'off' be 'null' by default in 'Unpacked.next'.

Add 'buffer_consumed' to 'Unpacked' which has to be invoked once we
write to 'buffer' with the number of bytes written.

Make 'data' in 'Unpacker' return an unowned 'Object' since it is held by
the unpacker.

msgpack.vapi

@@ -263,9 +263,11 @@ namespace MessagePack
 	public struct Unpacked
 	{
 		MessagePack.Object data;
-		MessagePack.Zone release_zone ();
+		[CCode (cname = "msgpack_unpacked_init")]
+		public Unpacked ();
+		public MessagePack.Zone release_zone ();
 		[CCode (cname = "msgpack_unpack_next")]
-		public MessagePack.UnpackReturn next ([CCode (array_length_type = "size_t")] uint8[] data, out size_t off);
+		public MessagePack.UnpackReturn next ([CCode (array_length_type = "size_t")] uint8[] data, out size_t? off = null);
 	}
 
 	public const size_t UNPACKER_INIT_BUFFER_SIZE;
@@ -287,9 +289,10 @@ namespace MessagePack
 			return buf;
 		}
 		public size_t buffer_capacity ();
+		public void buffer_consumed (size_t size);
 		public UnpackReturn next (out MessagePack.Unpacked pac);
 		public int execute ();
-		public MessagePack.Object data ();
+		public unowned MessagePack.Object data ();
 		public MessagePack.Zone release_zone ();
 		public void reset_zone ();
 		public void reset ();