Store references to other types inside a type class

[David Nolden]

Generally, the Data classes must only contain 'persistent' types that
can be copied freely between memory locations, and that stay valid
over multiple runs of the application, because the data is copied
directly to the harddisk.

You can use all types that start with "Indexed" plus all structures
that contain no pointers and only Indexed... members or primitive
types (eg. IndexedDeclaration, IndexedDUContext, IndexedString,
DeclarationId, ...)

Greetings, David