Changed several function names for slightly better consistency into the future, and and revised the design of not-yet-implemented syntaxes. Now private keys will be referred to by variable names.
Added isWfExternallyVisibleWord(), isWfKey(), checkKey(), and the (public ...) and (private ...) UserKnowledge syntaxes. Also, renamed getFreeVars() to getFreeVarsOfTerm() in preparation for getFreeVarsOfUserAction().
Added a "knownIsPrivateKey" field to all environment entries. For now, it's always unknown.