[sdiy] avrsyn update

[Daniel Kruszyna]

I've done some improvements to the avrsyn code since last time:

http://krue.net/avrsyn/

The two dcos now have individual level controls and are not
simply averaged before going to the filter. The mixer controls
use MIDI CC messages like the envelope controls.

I've also modified all of the code for control signals to run
at a constant interval, eliminating some 32x16 bit multiplies.
All 8bit multiplies have been converted to use the AVR's hardware
multiply instructions and the MUL8X8* routines have been removed.

Oh, and the key scanning routine runs as a state machine now and
doesn't require the "wait 10us" routine anymore.

So it's mainly internal changes except for the mixer, but they
should make it easier to add more features in the future.

By the way, are there any simple delay-based effects I could
implement? I figure there is enough spare ram for about 25ms
of audio storage.

-- Daniel