Blargg's saw wave DPCM IRQ demo in NSF2

Since NSF2 is now implemented in NSFPlay 2.4 beta 12, I decided to port one of the often cited examples of things that NSF2 would be good for to it.

Blargg's saw wave DPCM IRQ demo used the DPCM IRQ to generate saw waves and play a tune.

Here it is in NSF(2) form:
https://github.com/bbbradsmith/nes-audio-tests/raw/master/build/nsf2_saw_song.nsf

Source code at:
https://github.com/bbbradsmith/nes-audio-tests/blob/master/nsf2_saw_song.s

This is quite a significant development. The implementation of the things like SuperNSF and Deflemask drivers could become much better organized by using the cycle-IRQ feature.