Replace binarystruct with explicit marshal/unmarshal methods for packets

[Kangaroux]

Main reason for this is that the github.com/mixcode/binarystruct library does a ton of allocations and is quite slow compared to encoding/binary. It cuts down on boilerplate and is convenient for speeding up development but ultimately doing it manually is better. It would also be easier to test if the packet encoding/decoding is separate from the handlers.

Packets should have a Marshal and Unmarshal method:

type Packet interface {
    Marshal() ([]byte, error)
    Unmarshal(data []byte) error
}

• Refactor authd packets
• Refactor realmd packets