Thermal Rearrangement of 2,5-Bis(dicyanomethylene)bicyclo[4.2.0]oct-7-ene and 2,5-Bis(dicyanomethylene)bicyclo[4.2.0]octa-3,7-diene. Unexpected Formation of 2,6-Bis(dicyanomethylene)bicyclo[3.3.0]octa-1(5)-ene and -octa-3,7-diene, New Electron Acceptors