Initial fuzzing setup

[poljar]

This PR introduces an AFL based fuzzing setup for vodozemac.

Not many things are yet fuzzed, a comprehensive list of codepaths we'll want to fuzz:

• [x] Olm message decryption
• [x] Olm pre-key message decoding
• [x] Olm non-pre-key message decoding
• [ ] Account unpickling
• [ ] Session unpickling
• [ ] Olm unpickling from a libolm pickle
• [x] Megolm decryption
• [x] Megolm inbound session creation
• [x] Megolm session import
• [ ] Megolm unpickling
• [ ] Megolm unpickling from a libolm pickle
• [x] Megolm message decoding