Execute rule user code blocks when reducing the rule

spec/test_parser_rule_user_code.d

@@ -0,0 +1,14 @@
+import testparser;
+import std.stdio;
+
+int main()
+{
+    return 0;
+}
+
+unittest
+{
+    string input = "ab";
+    auto parser = new Testparser.Parser(cast(const(ubyte) *)input.ptr, input.length);
+    assert(parser.parse() == true);
+}