Unpack nested IteratorAggregate objects for Count

[jmikola]

From my previous comment in #2642 (comment):

Within the Traversable case, the function checks for an IteratorAggregate and attempts to unwrap it once by calling IteratorAggregate::getIterator(). The function then checks for a Generator instance and handles that separately. After that point, the existing implementation makes two assumptions, which I believe are premature:

1. It assumes that the object is an Iterator instance when it invokes the key() method. This fails to acknowledge that IteratorAggregate::getIterator() can return any Traversable instance.

This PR adds logic to unwrap nested IteratorAggregate objects until we encounter a Traversable that is not an IteratorAggregate instance. This ensures that generator detection (#2149) and internal Traversables (#2642) are properly detected before we attempt to restore the Iterator's position after counting (#1125).

In a separate commit, the PR also adds missing regression tests for Count's handling of non-nested IteratorAggregate objects.

[jmikola]

See: zend_user_it_get_new_iterator(). If IteratorAggregate::getIterator() returns an instance of the same class, PHP throws an exception. This is a basic attempt to prevent infinite recursion, although it is certainly still possible if two different IteratorAggregate implementations return instances of each other.

[codecov-io]

Codecov Report

Merging #2689 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2689      +/-   ##
============================================
+ Coverage     53.85%   53.87%   +0.01%     
  Complexity     2745     2745              
============================================
  Files           102      102              
  Lines          7193     7193              
============================================
+ Hits           3874     3875       +1     
+ Misses         3319     3318       -1

Impacted Files	Coverage Δ	Complexity Δ
src/Framework/Constraint/Count.php	88.88% <100%> (+2.77%)	16 <0> (ø)	⬇️

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[sebastianbergmann]

Thanks!