CombinationsSequence: underestimatedCount == count

[dabrahams]

Not doing this makes it needlessly costly to create an Array of the results.
It's a reasonable incremental improvement until you get around to #219, which would handle this automatically.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
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[pyrtsa]

Doesn't this conflict with the complexity requirement of Sequence.underestimatedCount?

  /// - Complexity: O(1), except if the sequence also conforms to `Collection`.
  ///   In this case, see the documentation of `Collection.underestimatedCount`.
  var underestimatedCount: Int { get }

The requirement is relaxed for non-random access collections, so not a problem for #219.

[dabrahams]

Hum, indeed. What is the complexity of your count? If it's log(N) IMO this is acceptable; complexity requirements are somewhat fungible (e.g. first on a lazy filtered collection is arguably not O(1)). Otherwise, I suppose not.

Oh, but they totally broke the complexity bound of underestimatedCount. It was O(n) before ea49459b7594. What a debacle.

[dabrahams]

Actually, that O(1) constraint is meaningless as outlined here. You should ignore it.

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test