Novel Phosphonium Cations. 2. Electrophilic Chemistry of Tetraphosphacubane: Novel Monophosphonium Ions of Ethylation, Benzylation, Acylation, and Adamantylation, Di- and Triphosphonium Ions of Acylation/Alkylation and Alkylation/Protonation, and Monoprotonation of Tetraoxo- and Tetrathioxotetraphosphacubane