* nx.tcl (substdefault): Unify and harden substdefault handling. Both, per-class and per-object substdefault should now behave similarly in absence of a pair of evaluation brackets. Also, an attempt is made to capture ill-formed input to subst earlier ([info complete]). An actual substdefault reform, however, must tackle the [subst] calls (-novariables?) and make the calls more robust (at the script and C level; [apply]-like?). Added some tests for documentation.
* nx.tcl (MetaSlot.parseParameterSpec): Refine handling of the type converter to expand unqualified names to the "nearer" namespace (i.e., namespace of the slot-owning object) rather than "". Along the way, intercept invalid type=* values earlier (empty string, "::"). Added some tests.