Issue 5024: Rework the Preinit framework into something simpler

Issue 5024: Rework the Preinit framework into something simpler

The previous introduction of the Preinit class had to work with
uninitialized data members in a state before construction.

Particularly for data structures like vector classes, this
was quite awkward.  Moving such structures into a separate
class initialized before the smobifying base class fixes
this deficiency and makes the behavior much more reliable,
at the cost of making the class hierarchy a bit more nested.