Implement dx21 style pitch EG

[vampirefrog]

[vampirefrog]

Add two timer modes: FM timer and PCM timer.

FM timer would use the chip's internal timers (A or B), and will be useful if fmtoy will run with real hardware. But we can use the chip emulation as well.

PCM timer will just use the sample rate to trigger timer events.

The timer events will be used to advance the LFO and the EG.

[vampirefrog]

Pseudo code for pcm timers:

struct timer {
  int numerator, denominator, remainder;
};
void timer_init(struct timer *t, int numerator, int denominator);
int timer_advance(struct timer *t, int num_samples, int *triggered);

void render(int num_samples) {
  int remaining_samples = num_samples;
  while(remaining_samples > 0) {
    int triggered = 0;
    int render_samples = timer_advance(t, remaining_samples, &triggered);
    remaining_samples -= render_samples;
    render_chip_samples(render_samples);
    if(triggered)
      lfo_tick()
  }
  
}