replace h3ToString

[dcooley]

to solve - SymbolixAU/h3lib#6

Remove call to h3ToString() and write our own version

SEXP h3ToSexpString(H3Index h3) {
  char str[17];
  //h3ToString(h3, str, sizeof(str));
  int res = snprintf(str, sizeof(str), "%" PRIx64, h3);
  return Rf_mkChar(str);
}