=head2 mro::get_isarev($classname)
Returns an arrayref of classes who are subclasses of the
-given classname. In other words, classes who we exist,
-however indirectly, in the @ISA inheritancy hierarchy of.
+given classname. In other words, classes in whose @ISA
+hierarchy we appear, no matter how indirectly.
This is much slower on pre-5.9.5 Perls with MRO::Compat
than it is on 5.9.5+, as it has to search the entire