Fix building in Xcode 11.4

[benasher44]

Changes in this pull request

This fixes an ambiguity with Swiift.Collection and a Collection typealias in DifferenceKit.

Checklist

• [x] All tests pass. Demo project builds and runs.
• [x] I added tests, an experiment, or detailed why my change isn't tested.
• [x] I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
• [x] I have reviewed the contributing guide