Reimplement java.lang.String with JS Strings.

Per those recent changes, these now need to be:

```
  var value = buffer.class.getField("I.value.[C").get(buffer);
  var count = buffer.class.getField("I.count.I").get(buffer);
```
(And the same change needs to be made to the override of *java/lang/StringBuffer.toString.()Ljava/lang/String;*).