[math] Another taste question about collection

[Benson Margulies]

The hash map classes define 'pairsSortedByValue'. In the case of a map
that delivers objects, this requires the value type to implement
comparable, which, of course, not all classes will. It seems insane to
only support these maps (e.g. OpenIntObjectHashMap<T>) for 'T extends
Comparable'. So I plan to make it throw if the type happens not to
implement comparable.