RangeSet should indicate if a mutation operation was a no-op

[dimo414]

RangeSet's mutation methods (add(), remove(), etc.) could leave the RangeSet unchanged (if rangeSet.encloses(range) or range.isEmpty()) but there's no easy way to detect when this occurs without manually inspecting the RangeSet or Range before the operation.

The equivalent methods on Set all return a boolean, returning true if the Set changed as a result of the operation; it would be useful if RangeSet behaved similarly.