private void grow() {
        // linear growth, as we should always be working on small sets
        entries = Arrays.copyOf(entries, entries.length + 10);
    }
Expose the index in the internal array of a given object. -1 is returned
if the object is not present in the internal array.

@param e
           an object to check whether exists in this set
@return the index of the argument e in the internal array, or -1 if the
        object is not present
vespajlib/src/main/java/com/yahoo/collections/TinyIdentitySet.java