Major semantics/knowledge refactoring

- Concepts are now just class declarations, and instances are instances
of those declared classes.
- Hence, knowledge.ontology is now just file with a bunch of child
Concept declarations
- Slots are now stored in separate properties to that they can be
properly inherited. But still they can be retrieved as a map using a
slots() convenience method.
- TMRs for a list of lemmas can be generated using the top level
semantics.tmr(lemmas) method