Send but only if we have the unique reference to the object
Note that implementing this trait has to be done especially careful.
It must only be implemented on types where the uniqueness of a reference
can be determined, i.e. the reference count field is accessible, and it
must only have references itself to other types that are
SendUnique is not enough for the other types unless uniqueness of
all of them can be guaranteed, which is e.g. not the case if there’s a
getter for them.