This module will contain CountedSet and potentially other multiset
implementations.

Similar to other collections in the package, CountedSet is backed by an internal _storage property, and exposes an initializer for preallocating storage.

Sequence conformance is implemented by iterating over the stored elements, repeating according to each stored count.

Collection conformance is implemented by using the index of an element in the underlying storage along with a position to distinguish between copies of the same element.

The implementation requires all elements to be unique and all multiplicities to be positive.

Tests have been added for initializing empty CountedSets with and without a minimum capacity.

Using unsigned integers for multiplicity of an element in a counted set makes it much clearer that negative values are not allowed.

Initializing a counted set with a dictionary literal can now be done with any given multiplicity. Negative values are now barred by the compiler, and multiplicities of zero can simply be discarded.

This change necessitated a slight tweak to the way iteration is performed: since Swift.repeatElement(_:count:) requires Int counts, multiple calls are used for multiplicities larger than Int.max.

CountedSet.count will trap on cardinalities larger than Int.max, as with most Sequences.

The custom implementation of count adds the multiplicities of the elements together, which is far more efficient than the default approach of iterating through the set manually.

underestimatedCount has also been customized: it exposes the count of the underlying dictionary, which is guaranteed to be less than or equal to the cardinality of the counted set itself. This may be used to avoid trapping for sets with very large multiplicities.

Each requirement has been implemented according to the mathematical definitions, complete with careful documentation to ensure users understand how that may diverge from expectations.

Because Swift.Dictionary does not specify time complexity for every method being used, part of the implementation cannot either.

Tests have been added to exercise all of the SetAlgebra implementation, including each of the axioms required by SetAlgebra.

This operation adds the multiplicities of each element together.

A test has been added to cover it.