#248 resid bitfade

[Leandro Nini]

This is a rough implementation of the shift register and wavefom zero bitfade, as mentioned in [bugs:#1318] and [bugs:#1037].
It is not a realistic model as I don't have enough reference data and moreover I don't see any practical use for this, so it's not worth the trouble.
One thing that I've noticed though is that the R4AR have longer fading times, almost near the 8580 ones, not sure if this is true for all the chips from this revision.

[gpz]

applied in r38860, thanks!

can we close #1318 now?

[Leandro Nini]

Sure!