Index TypedefNamespace

We have introduced TypedefNamespace as a placeholder for the TypeEffectiveStatement and TypedefEffectiveStatement relationship.

Lookups in this namespace do not work, though. We need to retrofit abstract classes with support for this namespace.

This either needs to be an explicit index, just like schema tree is, or it needs to perform a substatement search. The latter is probably more feasible given that typedefs can appear in a number of places – but that remains to be seen.