Isolate compatibility checker into standalone function

[sdboyer]

The specific algorithm used for checking backwards compatibility between two thema schemas should be isolated into a standalone function. Initially this can live under internal/ but it may be worth exposing it in an invariants subpackage or something.

Simple subsumption is currently insufficient for our purposes (though that will need re-investigation post- #82), but even if/when this is not the case, it's worth being clear about the algorithm Thema uses for this.

Some steps already taken here.