Added option lazy_compute_serialized_size

This optional arg (default to false) changes the behavior of  to be lazy
instead of def, The main advantage is HUGE speed increase when serializing nested objects.
On the other hand, an object can not be serialized again(!) after it has been modified.

scalabuff-compiler/src/main/net/sandrogrzicic/scalabuff/compiler/Generator.scala

@@ -25,6 +25,8 @@ class Generator protected (sourceName: String, importedSymbols: Map[String, Impo
    */
   protected var optimizeForSpeed = true
 
+  protected var lazyGetSerializedSize = false
+
   /**
    * Generates the Scala class code.
    */
@@ -222,7 +224,9 @@ class Generator protected (sourceName: String, importedSymbols: Map[String, Impo
       out.append(indent1).append("}\n")
 
       // getSerializedSize
-      out.append("\n").append(indent1).append("def getSerializedSize = {\n")
+      val definition = if (lazyGetSerializedSize) "lazy val" else "def"
+
+      out.append("\n").append(indent1).append(definition).append(" getSerializedSize = {\n")
         .append(indent2).append("import com.google.protobuf.CodedOutputStream._\n")
         .append(indent2).append("var __size = 0\n")
       for (field <- fields) {
@@ -529,6 +533,11 @@ class Generator protected (sourceName: String, importedSymbols: Map[String, Impo
             case OptionValue(key, value) => key match {
               case "java_package"         => packageName = value.stripQuotes
               case "java_outer_classname" => className = value.stripQuotes
+              case "lazy_compute_serialized_size" => value match {
+                case "true"         => lazyGetSerializedSize = true
+                case "false"        => lazyGetSerializedSize = false
+                case _              => throw new InvalidOptionValueException(key, value)
+              }
               case "optimize_for" => value match {
                 case "SPEED"        => optimizeForSpeed = true
                 case "CODE_SIZE"    => optimizeForSpeed = false