[OrderedDictionary] mapValues with key context

[tayloraswift]

implementation for swiftlang/swift-evolution#3060

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• 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.

[dnadoba]

This looks very good. Thank you!
swift-collections is not governed by swift-evolution and this change can be independently reviewed and merged.
With that being said, it would be great if we end up with the same name.
I think as your swift-evolution pitch for Dictionary is still moving we should still wait unless this get's stuck. This might already be what you had in mind as this is a Draft PR.