Add support to return void in a function

* statement `return;` is valid now.
* functions return void by default, an explicit return is not necessary now

ZenScript/src/main/java/stanhebben/zenscript/expression/ExpressionJavaLambda.java

@@ -90,6 +90,7 @@ public class ExpressionJavaLambda extends Expression {
 		for (Statement statement : statements) {
 			statement.compile(environmentMethod);
 		}
+		output.ret();
 		output.end();
 
 		environment.putClass(clsName, cw.toByteArray());