Introduce class ipl\Stdlib\CallbackFilterIterator

[nilmerg]

This class stems from the motivation to have a filter iterator that uses callbacks, which behaves like any other iterator. Just like the SPL's CallbackFilterIterator, but that works differently: It isn't valid unless explicitly rewound first.

Any other iterator, especially generators, doesn't need to be rewound first. That's most obvious with a generator that yields conditionally, just like a filter iterator would.

Any call to valid(), key() or current() should return the same result as the very first iteration step would.

[nilmerg]

I refuse to put the errors reported by phpstan into the baseline. I expect phpstan to not complain about these on 7.x if it doesn't on 8.x -.-

[nilmerg]

It may optionally be sufficient to really call rewind() explicitly where required. Which wouldn't necessitate this custom implementation. Though, I still wonder why generators behave differently to the CallbackFilterIterator. A generator isn't just one upon reaching the first yield at runtime, it's one as PHP detects that yield is used upon parsing. So it's not much of a difference to an iterator which is created at runtime (Iike a generator as closure). But it doesn't need to be rewound first, before a call to valid(), key() or current().

So, yeah, it's an edge-case, but maybe one that pops up in the future again? And then there may be indeed the requirement for a method parameter that accepts generators and ((callback)filter)iterators.