This iterator can be used to iterate across strings in a cross-platform way. It handles surrogate pairs on platforms that require it. On each iteration, it returns the next character offset as key and the next character code as value.
Note that in the general case, because of surrogate pairs, the key values
should not be used as offsets for various String API operations. For the
same reason, the last key value returned might be less than
s.length - 1.