[swfinterp] Implement String basics

test/swftests/StringBasics.as

@@ -0,0 +1,11 @@
+// input: []
+// output: 3
+
+package {
+public class StringBasics {
+    public static function main():int{
+        var s:String = "abc";
+        return s.length;
+    }
+}
+}