Forks break assumptions for 'convergent' imports

[povik]

The convergent import of mutable variables (implemented with VarImplMutable._attempt_simple_import) is based on the assumption that for any pair of nodes in the universal cover of the immutlinks graph, either there is a mutlinks path from one node to the other, or the ordering of the two nodes is undefined (and we are free to do anything in forwarding of mutable variables between the two nodes). This assumption breaks with execution forks, since nodes in the two threads of execution downstream of the fork don't have the property vis-a-vis each other.