- previous code used derived string iterators, but these would
  be largely ignored anyhow since the underlying std::sub_match
  str() method would just yields a std::string anyhow.

  The SubStrings::str(size_t) method wasn't used in any code, so now
  just use std::string iterators only.

  This change simplfies overall handling, since it removes an unneeded
  template dependency.