Increase phpstan level from 7 to 8

- Fixed all phpstan errors and ignoreds.
 - False positives now have a "Why:" comment on phpstan config.

library/Exceptions/RecursiveExceptionIterator.php

@@ -16,6 +16,8 @@ use UnexpectedValueException;
 
 /**
  * @author Henrique Moody <henriquemoody@gmail.com>
+ *
+ * @implements RecursiveIterator<int, ValidationException>
  */
 final class RecursiveExceptionIterator implements RecursiveIterator, Countable
 {