The Orderable interface represents a value that can be linearly ordered.

Currently only supports linear (<, =, <=) operations. Eventually we may want to do other types of orderings, in which case there would probably be a number of interfaces for each "class" of ordering.

The implementation must handle the comparison of null values. This may require some changes to the interface, since (at least in some contexts) comparing a value with null should return unknown instead of true or false.