Care about refcounts when reclaiming memory allocated for static objects.

Care about refcounts when reclaiming memory allocated for static objects.

With commit aff357309f6 we started freeing numeric flyweights when the library
usage count drops to zero. But that triggers assertions in basic::~basic
when compiled with -DDO_GINAC_ASSERT because these flyweights are still being
used by their matching ex objects (their refcount is still 1). Instead of
removing that assertion, let's just call the ex::~ex manually and let it do
the freeing.