Comparator combinators and extension methods

Add the following methods in the new class Comparators:
Comparators.reverseOrder()
Comparators.naturalOrder()
Comparators.reverseOrder(comparator)

Comparators.comparing(Mapper<R,T>)
Comparators.comparing({Int,Long,Double}Mapper<T>)

Add the following extension methods on Comparator:
Comparator.reverse()
Comparator.compose(other)