canonically sort diff results

[jclerman]

Currently, the individual elements of rendered diffs do not seem to be sorted. That has impacts for robot (see ontodev/robot#1192), whose diff-results have some inconsistency (not in semantic meaning of course, but in terms of literal reproducibility).

Readability of results is also somewhat impaired, since, for example, for a class, differences in subclass axioms may appear mixed in with annotation differences, rather than in a consistent location (top of the list, say).

The code that attempts to sort results appears to be here. Not sure that it's working as intended though.

I also note that this library doesn't have a test-suite - adding one would facilitate contributions (and have the usual benefits, of course).

[balhoff]

I agree the output should be consistent. Also agree about tests. I will try to get to these changes soon, but right now not sure when that will be.