Sound Banks Should Allow More Than 3 Samples Per Instrument

This could be useful because:
- Some instruments (e.g. piano) sound terrible split into just 3 samples, making their use a headache while potentially sacrificing MIDI channel space and additional instrument data
- Eventual sf2->sm64 converter would be much more practical

This is not a small change; both the conversion tools and a decent portion of the audio code would need modified for this to work. As long as the 3 instrument cap isn't partially imposed by libultra, this should be possible. Performance could technically be made worse with this if implemented poorly but probably not by much.