Provide some API to access the currently-defined types

Right now there is no "publicly" exposed function for inquiring as to the types available in a typed-function instance. (There is just `._findType` exposed "for unit testing only".) There should be some standardized public API for asking about the types present, perhaps at least read-only access to the `typeList`, or some other API if it would be more sensible. It might also be nice to be able to ask a typed-function if an entity is in some type, in its opinion.