Work on real D strings instead of ubyte pointer and length

Also fix a couple UTF-8 decoder bugs!

spec/test_parser_rule_user_code.d

@@ -9,6 +9,6 @@ int main()
 unittest
 {
     string input = "ab";
-    auto parser = new Testparser.Parser(cast(const(ubyte) *)input.ptr, input.length);
+    auto parser = new Testparser.Parser(input);
     assert(parser.parse() == true);
 }