Moved from *.__del__() to exclusive use of *.cleanup().

Moved from *.__del__() to exclusive use of *.cleanup().

*.__del__() is run some unspecified time after the refcount for an
object is reduced to zero.  Sometimes that means that the rest of the
world has already been deallocated, which makes life difficult,
especially when Python won't attempt to construct stack traces inside
*.__del__().  We were always (hopefully ;) calling del(*) anyway,
so we just replace those calls with *.cleanup()