Matrix multiplication for the persistent vector implementation

This is a simple implementation of matrix multiplication for the persistent vector implementation.  It's just the linear algebra textbook method -- I don't think it's worth doing something like Strassen's method, since if we really want efficiency we would plug into one of the java implementations.  (Also, as far as I know, Strassen only starts to be worth it on really large matrices; wikipedia says dimensions of more than 1000).