void init(String s) {
        this.s = s;
        left = s.length();
        this.upto = 0;
    }
Used by DocumentsWriter to implemented a StringReader that can be reset to a
new string; we use this when tokenizing the string value from a Field.
stroom-index/stroom-index-lucene553/src/test/java/stroom/index/lucene553/analyser/ReusableStringReader.java