Mqtt message decoding/encoding library

This is a library designed to be used for creating mqtt clients or mqtt brokers. As many things as were reasonable are encoded in the type system, e.g. - packets have their own types, except for PUBCOMP/PUBREC/PUBREL/PUBACK, since they are essentially the same - reason codes are enums and it's not possible to build a packet with an invalid reason code - properties are defined for every single packet type and therefore only valid property codes can be written into the packet

Supports: MQTTv3 and MQTTv5

Messages of these versions should be decodable/encodable with this library.

What works so far (Implemented and tested):

| Encode | Decode | Packet Type | |--------|--------|-------------| | ✅ | ✅ | Connect | | ✅ | ✅ | Connack | | ✅ | ✅ | Publish | | ✅ | ✅ | Puback | | ✅ | ✅ | Pubrec | | ✅ | ✅ | Pubrel | | ✅ | ✅ | Pubcomp | | ✅ | ✅ | Subscribe | | ✅ | ✅ | Suback | | ✅ | ✅ | Unsubscribe | | ✅ | ✅ | Unsuback | | ✅ | ✅ | Pingreq | | ✅ | ✅ | Pingresp | | ✅ | ✅ | Disconnect | | ✅ | ✅ | Auth |

However certain things still need to be added/improved:

• [ ] A better command building API?
• [ ] Make only necessary code public
• [ ] Support for Maximum Packet Size (MQTTv5). Should not send certain properties if they "bloat" the packet
• [ ] Ensure all properties have the correct Optionality set in their types
• [ ] Add some fuzzing tests to prevent unwanted panic! calls
• [ ] Improve documentation