Collection comparer without order

[StephenCleary]

Currently EquateSequence requires elements to be in corresponding order. This issue is to add some kind of support for a collection equality comparer that ignores ordering of elements. This will require some kind of state, so this will be an expensive comparer to use.

The comparer will need to allow a custom element equality comparer, one that may have special null handling.

High-level algorithm:

• If counts (soft reification to ICollection / ICollection<T> / IReadOnlyCollection<T>) aren't equal, then false. If both counts are 0, then true.
• Determine cardinality of equivalence classes, and test that both collections have the same cardinality for equivalent items.

Not sure how to handle GetHashCode, since there's no order defined for the items nor their equivalence classes. Any hash code combination would have to use a commutative combiner, which is not currently in the library.

[StephenCleary]

Alternative:

• Reify one of the collections to a new List<T> and walk the other collection, removing an equivalent element each time. Downside: O(N^2) instead of O(N). Shouldly and FluentAssertions both take this approach.

[StephenCleary]

Alternative:

• Walk both collections simultaneously, keeping two List<T> for unmatched elements. Also O(N^2) in the worst case (all elements unique), but only O(N) for collections that happen to be in a compatible order.

[StephenCleary]

The commutative combiner could just add hash codes along with a constant value like 65536 to act as a quasi element counter.

[StephenCleary]

The equivalence-class-count approach is fastest (O(N)), but is complicated by the fact that nulls can be items in the same equivalence class as non-null items. Wondering if we can use Dictionary<Wrapper<T>> for some kind of struct Wrapper<T> type.