Add SortedMaps.mutable.empty(Comparator) and SortedSets.mutable.empty(Comparator).

[motlin]

trafficstars

We're missing symmetry. This exists and works:

SortedBags.mutable.empty(comparator);

But these don't exist.

SortedMaps.mutable.empty(comparator);
SortedSets.mutable.empty(comparator);

[Desislav-Petrov]

I can take that if u don't mind