New Soluble, Coplanar Poly(naphtahene-2,6-diyl)-Type p-Conjugated Polymer, Poly(pyrimido[5,4-d]pyrimidine-2,5-diyl), with Nitrogen Atoms at All of the o-Positions, Synthesis, Solid Structure, Optical Properties, Self-Assembling Plenamena, and Redox Behavior