optionally store all objects in the same file

Previously each objects was stored in it's own file using the
pretty-printed representation of it's constructor.  Now it is possibly
to instead (or additionally) save all objects in the same file using
the object vector.

When all objects are stored in the same file then that is used by
default when the registry is initialized.  This is much faster than
reading and evaluating one file per object.

On the other hand bypassing the constructor makes it impossible to
modify the class of the stored objects.  For that reason it is still
possible to store each object individually.