10.11.1: XPClass::getDeclaredFields fix

Bugfixes

• Fixed lang.XPClass::getDeclaredFields() throwing an exception - @thekid
• Fixed issue #276: PHP 8.1: offset* / getIterator signature. Made codebase
  compatible with PHP 8.1 (while keeping PHP 7 support!) by adding the
  #[ReturnTypeWillChange] in relevant places.
  (@thekid)