Sequencer simplified, basically working.
Fixed bug in uninitialized karplus data.
Rewrote many routines in sequencer, reduced the audio facing
interface to one function:
event* seq_advance(int max, int* used);
which advances the sequencer a maximum of max samples, fills in
used with how many samples it actually advanced, and returns the
next control event or NULL if the next control event does not exist
or will occur max samples from now or more. If used is zero, the
synth continues calling for more control events.