Overhaul MIDI-handling code

[oubiwann]

All the old tests have been commented out, due to the breaking changes with the new datatypes and message parsing / handling. Additionally, old MIDI dispatch code has been commented-out and needs to be re-implemented.

Tasks:

• Create a new midi/messages.go MIDI struct
• Create utility functions for extracting both batch and non-batch MIDI messages
• Add methods for the easy extraction of MIDI data from the struct
• Re-enable the tests and refactor to use the new struct and methods
• Devise a new dispatch mechanism the utilises the new struct and methods