9.0.1: PHP 8.1 compatibility

• Made compatible with PHP 8.1 - add ReturnTypeWillChange attributes to
  iterator, see https://wiki.php.net/rfc/internal_method_return_types
• Replaced xp::errorAt() call with a less-expensive check on iconv() return
  value. See xp-framework/core#299
  (@thekid)