Apparent concurrent acetylene-vinylidenecarbene rearrangements, silyl-acetylide metathesis, and alkyne cleavage in the interaction of bis(trimethylsilyl)acetylene with (.eta.5-C5H5)Co(CO)2. Crystal and molecular structure of a novel biscarbyne complex: [.mu.3.eta.1-CSi(CH3)3][.mu.3.eta.1-C3Si(CH3)3][(.eta.5-C5H5)Co]3