The IsPublic method will return 'true' if the special public collection
defined in the ActivityPub spec is addressed in the 'to', 'bto' 'cc', or
'bcc' properties of the Object or any of the types extending from them.
This condenses the N types and M properties from N*M intermediate type
definitions to just M intermediate type definitions. These intermediate
types are no longer generated in the gen_<TYPE>.go files, but are all
within the separate gen_intermediate.go file.
This will hopefully reduce resource consumption during compilation of
the vocab package.
