Cycloaddition between Electron-Deficient π-Systems, Photochemical and Radical-Induced Reactions: A Novel, General, and Stereoselective Route to Polyfunctionalized Bridged Bicyclo[2.2.2]octanes, Bicyclo[3.3.0]octanes, Bicyclo[4.2.0]octanes, and Tricyclo[4.3.1.0 3,7 ]decanes