Added snapshot benchmarking script.
Added snapshot benchmarking script.

This script provides snapshot times with cold and warm caches,
serialization and deserialization times, serialized size, and validates
that the deserialized snapshot is equivalent to the original.

Initial results show that the stable_marshal extension slows down
serialization by a factor of 2-3 over the standard reflection-based
Protocol Buffers serialization, but deserialization time is reduced by
an equivalent factor. Since stable marshalling is necessary, there's not
much we can do, but marhsalling in general seems sufficiently fast at
the moment that I'm not concerned.

havoc-io/mutagen,havoc-io/mutagen