Implement Arbitrary

[killercup]

We want to generate random but valid messages.

• Implement the arbitrary by hand (so we don't add too many dependencies)
• Generate arbitrary float values
• Put the implementation behind a feature flag
• Add a roundtrip test

cf. #10 (reply in thread)

[marcelbuesing]

Probably needs to be put behind feature flags for now.

rust-fuzz/arbitrary#74