Add BUILD rules for fuzz tests

[russellhancox]

We have fuzz tests in the repo but they're not buildable by the existing BUILD rules (they were originally added with cmake rules but those got out of date quickly).

https://github.com/bazelbuild/rules_fuzzing is a good start though it doesn't support macOS out of the box.

[pmarkowsky]

Minor update on this.

After including the rules_fuzzing, the main blocker, is that libfuzzer isn't packaged with XCode. We can work around this by brew installing llvm in the GitHub VMs.

Past that we need to update the fuzzing tests source, create BUILD files etc.

[pmarkowsky]

@kallsyms figured out that we can just install libfuzzer in the local xcode and it works. #976 does this and uses rules_fuzzing.