No testing actually done yet.

I.e., don't let cell ordering depending on GHashTable.

This should handle running introspection in-tree too.  g_get_prgname
is not reliable for that, so pass in a separate environment variable.

Namespace, introspection, etc.

I'm flying blind here.  Code isn't yet tested.

(I.e., things didn't work and crashes left and right.  That sort of thing.)

This clarifies ownership for all GnmFunc members except "help" which
is in sight.

Change GnmFunc from a boxed type with weird ref-counting to a proper
GObject with normal ref-counting, but also usage counting.

Function descriptor tables in plugins lose two pointers that almost
nobody was using: usage notifer (use signal notify::in-use instead) and
the link notifying (use signal link-dep instead).

There is still room for plenty of cleanup here.

This happened with an active region.

Also add DERIV function, but only during the test suite.

This fixes the handler's lifetime annotation and adds introtrospection
for the rest of the derivative api.

Also fixes a cut-and-paste problem affecting derivative of a reference
to a non-existing cell.

Rework things to make scope of handler right.