Use bugs to measure fuzzer performance

My view of fuzzers is that "better" fuzzers can find more exploitable bugs than worse fuzzers. While coverage is a decent, easy-to-use, proxy for this, we should look into using crashes to determine fuzzer performance.

We already save the crashes found in each cycle/corpus snapshot.

Here's what's left to do that I can think of:
1. Implementing a method for identifying crashes. ClusterFuzz's method (similar to stack hashing) works pretty well in practice.

2. Finding a way to rank fuzzers based on the crashes they find.