Fuzz-Testing

To provide greater assurance in the correctness and security of Subnets, we should include fuzz-testing as part of our development lifecycle.

The goal of this issue is to get at least one fuzz-test running as part of Continuous Integration, so we have ongoing coverage. We can iterate this fuzz testing work in the immediate future to cover more of the subnets project.

## Milestones

- [x] Integrated with `cargo fuzz` without adding `arbitrary` to other modules
- [x] Identify at least one meaningful fuzzing target
- [x] Study the hot path for smart contracts for good candidates
- [ ] Used automated tools, e.g. [siderophile](https://github.com/trailofbits/siderophile), to identify other candidates
* Delayed due to LLVM compatibility issues that the maintainer is currently working to fix
- [x] GitHub Actions script that runs `cargo fuzz` in CI
- [ ] Verify that fuzz test actually runs in GitHub Actions for new pull requests and pushes
- [ ] Ensure fuzzing scaffolding is easy to extend as subnets development continues

### Stretch Goals

- [x] Parallelize CI fuzzing script
- [ ] Integrate with afl-fuzz to generate smart contracts for long-running fuzzing processes