Improve performance of `count` on `Combinations`

The implementation of the `counts` property on `Combinations` can be optimized to make use of the fact that the binomial is symmetrical around N/2 or (N - 1)/2.

More details available in [this review comment](https://github.com/apple/swift-algorithms/pull/51#discussion_r552056051) on https://github.com/apple/swift-algorithms/pull/51.

Read the Combinations implementation, focusing on the counts property, and review the linked discussion on pull request #51 for the optimization context. Use the binomial symmetry described in the issue, then verify that count results remain correct while the implementation performs less work.

Written by the indexing model from the issue text.