* Calendar Sequence API
* Add Calendar_Gregorian support and placeholders for dayOfYear
* Address review feedback and fix test failure
* Disable day of year for in-progress Gregorian calendar implementation
* Adjust the Calendar_Gregorian use of dayOfYear pending some more work there
* Remove if available FoundationPreview 0.4 checks (see #384)
* WIP: Implement `date(byAdding components: DateComponents, to date: Date, wrappingComponents: Bool)`
This largely follows ICU's calendar::add and calendar::roll.
* It takes around a minute to run these compatibility tests as of this moment. I expect it to take even longer as we add more tests to it. Disable them on CI for now. The appropriate tests should assert their expected absolute values respectively.
* Add a convenience initializer to Date.Components and use it in tests
