Make the file format more diff-friendly

[mtkennerly]

This is a proof-of-concept for #70 that tries to be a fairly minimal change.

Before:

3,0,0,0,120,16,4,2,0,0,0,128,0,256,128,0,0,128,0,256,2,0,1,0,0,14,52,1,0,0,52,1,1,0,52,1,2,0,57,1,3,0,60,1,4,0,65,1,6,0,67,1,8,0,57,1,8,0,62,1,11,0,59,1,10,0,55,1,12,0,55,1,13,0,55,1,14,0,62,1,8,0,0,0,1,1,0,8,67,1,3,0,67,1,7,0,67,1,11,0,69,1,2,0,64,1,5,0,65,1,6,0,64,1,9,0,65,1,13,0,0,1,0,1,0,1,-1,-1,-1,-1,-1,-1,

After:

4,

// general
0,0,0,120,16,4,

// instruments
2,
0,0,0,128,0,256,
128,0,0,128,0,256,

// patterns
2,
0,1,0,0,14,52,1,0,0,52,1,1,0,52,1,2,0,57,1,3,0,60,1,4,0,65,1,6,0,67,1,8,0,57,1,8,0,62,1,11,0,59,1,10,0,55,1,12,0,55,1,13,0,55,1,14,0,62,1,8,0,0,
0,1,1,0,8,67,1,3,0,67,1,7,0,67,1,11,0,69,1,2,0,64,1,5,0,65,1,6,0,64,1,9,0,65,1,13,0,0,

// arrangements
1,0,1,
0,1,-1,-1,-1,-1,-1,-1,