Refactor namings of serializers etc.

Source/MQTTnet/Internal/ArraySegmentExtensions.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace MQTTnet.Internal
+{
+    public static class ArraySegmentExtensions
+    {
+        public static byte[] ToArray(this ArraySegment<byte> source)
+        {
+            if (source.Array == null)
+            {
+                return null;
+            }
+
+            var buffer = new byte[source.Count];
+            if (buffer.Length > 0)
+            {
+                Array.Copy(source.Array, source.Offset, buffer, 0, buffer.Length);
+            }
+
+            return buffer;
+        }
+    }
+}