(str) add support for iterating over the codepoints in a string

[adapted from Josh and Neal's "cooking with Tiger" talk]

Placing the enclosed adapter method in java.lang.String enables clients to
iterate through the code points in a String very easily:

    void f(String s) {
for (int ch : s.codePoints())
;// do something with the codepoint ch here.
    }

Here is the proposed addition to java.lang.String:

public Iterable<Integer> codePoints() {
    return new Iterable<Integer>() {
        public Iterator<Integer> iterator() {
            return new Iterator<Integer>() {
                int nextIndex = 0;
                public boolean hasNext() {
                    return nextIndex < length();
                }
                public Integer next() {
                    int result = codePointAt(nextIndex);
                    nextIndex += Character.charCount(result);
                    return result;
                }
                public void remove() {
                    throw new UnsupportedOperationException();
                }
            };
        }
    };
}