Read payload bytes from individual sysex packets

[BenLeadbetter]

A valid use case might be to handle sysex messages packet by packet, rather than whole messages. In which case the client would need a method to retrieve the payload from an individual packet, so they can buffer the payload manually, for example.

This work should add a payload method to both the sysex7::Packet and the sysex8::Packet.

Each new method should return an iterative to over the packet's payload bytes. Unlike with the message payload iterative, these will be constant in the their nth implementation.