PHP 8.1 return type deprecation notices

[Bilge]

Further to #97, there are still more deprecation warnings for PHP 8.1.

User Deprecated: Method "IteratorAggregate::getIterator()" might add "\Traversable" as a native return type declaration in the future. Do the same in implementation "Ds\Vector" now to avoid errors or add an explicit @return annotation to suppress this message.
User Deprecated: Method "ArrayAccess::offsetSet()" might add "void" as a native return type declaration in the future. Do the same in implementation "Ds\Vector" now to avoid errors or add an explicit @return annotation to suppress this message.
User Deprecated: Method "ArrayAccess::offsetGet()" might add "mixed" as a native return type declaration in the future. Do the same in implementation "Ds\Vector" now to avoid errors or add an explicit @return annotation to suppress this message.
User Deprecated: Method "ArrayAccess::offsetUnset()" might add "void" as a native return type declaration in the future. Do the same in implementation "Ds\Vector" now to avoid errors or add an explicit @return annotation to suppress this message.
User Deprecated: Method "ArrayAccess::offsetExists()" might add "bool" as a native return type declaration in the future. Do the same in implementation "Ds\Vector" now to avoid errors or add an explicit @return annotation to suppress this message.
User Deprecated: Method "IteratorAggregate::getIterator()" might add "\Traversable" as a native return type declaration in the future. Do the same in implementation "Ds\PriorityQueue" now to avoid errors or add an explicit @return annotation to suppress this message.