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):

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