Handle references in keys of StrictHashSet/StrictHashMap

[TysonAndre]

• If arrays contain references, then use copies of the arrays that don't contain references, instead (and use copies for parents of those arrays, as well). If there are multiple occurrences of the original array then use the same copy every time.
• Alternately, throw a RuntimeException (e.g. for reference cycles)

$x = new Teds\StrictHashSet();
$v = 1;
$a = [&$v];
$x->add($a); // should add a copy of the array without references, instead