DSP: add method to kill dsp thread

As the dsp-thread can sometimes become unresponsive.
Thread->stop is deprecated, but that's a final resort anyways.
Instrument: optimize instruments interface and implementation

- definst takes a function producing a closure now
- don't rewrite instrument, rewrite note kernel only
- don't call the function with freq, amp, dur as they are fixed anyways
Patterns: use midi pitches instead of frequencies
Project: update to clojure 1.5.1
UGens: some optimizations

- remove unnecessary type hints
- precalculate things where possible
- misc. other optimizations
Utils: add type hints

to avoid reflection