Move drum and guitar patterns into modular files

[cass-dlcm]

With my midi-gen-py project, I made it so that the patterns are stored in JSON files and are read in. This allows the user to define their own patterns easily. I wish to replicate that feature in this project.

[cass-dlcm]

Decided that they won't be stored in json, but instead in a simplified version of MIDI.

Details:

• no track or file headers, just a file length indicator stored as a vlq and a ticks per measure stored as a vlq
• only events are note on, note off, end of track, repeat segment
• note events stored using the vlq for number of ticks, a byte to tell event type with channel, and pitch
• velocity is not included as that's set by the program
• pitches stored in guitar patterns are indices, while pitches stored in drum patterns are actual pitches
• repeat segment events specify when the repeat starts, how long the repeat is in the file, and how many times to repeat
• repeat format: TT (TT TT TT) 0C NN (NN NN NN) LL (LL LL LL)
• TT (TT TT TT) is a vlq for the number of ticks from the previous event
• 0C is the event indicator, c being the channel
• NN (NN NN NN) is the number of repeats as a vlq
• LL (LL LL LL) is the length in bytes as a vlq of the repeat segment
• repeat segments can be nested