Instrument loading

[200sc]

dls files and sf2 files are probably the first targets for loading in instruments, to then determine how we can fit those instruments into our wave function syntax.

[200sc]

DLS files have been parsed, we just need a structure to parse their instruments into. Idea:

type Instrument interface {
    GetSample(Pitch) Audio
}

This covers the common instrument case of being a set of samples where specific samples should be used for specifc pitches. Drumkits would need to be separate.