Synthesise multiple MIDI instruments, add instrument bank

[gyng]

Right now make_midi_samples takes in one instrument, and the next step is for it to take in a HashMap of instruments along with a default.

Might be easier if we finally cave in and Box our closures (#7), since working with higher-order closures is a giant pain due to unsized-ness. It might also let us pass in JsClosures

After this then we can even begin to talk about MIDI FM sample synthesis (and that then needs phase adjustment).

[gyng]

A sampler generator was added, but the API design is not great for building sample banks.