[HashTreeCollections] Add `TreeDictionary.combining(_:by:)`

[lorentey]

This adds an elaborate new entry point for structurally combining two TreeDictionary instances, complete with a deeply customizable element-wise combining strategy.

This PR is a proof of concept that may turn into a shipping feature. It currently builds, but it has not yet seen any testing whatsoever. The new APIs currently require Swift 5.7 or better, as they use primary associated types.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [ ] I've added benchmarks covering new functionality (if appropriate).
• [ ] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [ ] I've updated the documentation if necessary.

[lorentey]

Rescheduling to 1.2.