ClassName is its canonical (?) source-syntax (dotted) name, e.g. 'java.util.Date'

NB not used outside jni_junk and jpl_test (is this (still) true?)

NB oughta use the available caches (but their indexing doesn't suit)

TODO This shouldn't exist as we have jpl_class_to_entityname/2 ???

The implementation actually just calls Class.getName() to get the entity name (dotted name)