Specify precision for collapsing range

[sffc]

There was an interesting bug that came in when implementing fractionalSecondDigits:

https://bugs.chromium.org/p/v8/issues/detail?id=9474

It seems that formatRange should specify the case when dates collapse. That should not be locale-dependent. For example, if the two dates appear different when formatted alone, then the range should not collapse.

Note: The input dates could be different, and the range could still collapse, if the fields to be displayed are less precise than the difference between the dates. The criterion for collapsing should consider the dates after fields have been applied, similar to rounding a number.

[sffc]

I think we should wait for user feedback and revisit this in the future with a new formatting option.