Function rule_vandnot

* Dest must exist. * We use this to update existing vectors, so it has to work if the dest and * src1 are the same. It's easy in the naive implementation, but trickier in * the mpz because you can't do the AND and NOT at the same time. We benchmarked * allocating a temporary against doing this in 3 ops without the temporary and * the temporary is significantly faster (for large vectors). */