src/{class-,}utilities.lisp: Add machinery for showing inheritance paths.

src/{class-,}utilities.lisp: Add machinery for showing inheritance paths.

To the general utilities collection, we add an implementation of
Dijkstra's distinguished-point shortest-path algorithm.

To the class utilities, we add a new type and function for reporting
inheritance paths, with the notion that this will be useful when
debugging problems where classes turn out to be incompatible with each
other for various reasons.