Improve u16::carrying_mul harness performance (model-checking#230)

This harness takes between 22-25 minutes in CI. Change the harnesses's
macro to use kissat solver instead, which brings verification time down
to ~7 seconds.

As an aside, I noticed this problem because the partition 1 runner in
model-checking#229 is taking ~55 minutes, where the other partitions are taking ~30
minutes. This harness accounts for almost the entire difference. One
nice consequence of partitioning verification is that it will make
performance issues like this more noticeable.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Author: Carolyn Zech

library/core/src/num/mod.rs

@@ -1759,6 +1759,7 @@ mod verify {
         ($type:ty, $wide_type:ty, $($harness_name:ident, $min:expr, $max:expr),+) => {
             $(
                 #[kani::proof]
+                #[kani::solver(kissat)]
                 pub fn $harness_name() {
                     let lhs: $type = kani::any::<$type>();
                     let rhs: $type = kani::any::<$type>();