support for storing samplepacks in ADPCM format

[tehKaiN]

now that mod_tool can merge samples from bunch of .mod files and store them in separate samplepack file and rest of .mod data separately, perhaps it'd be wise to store samples in ADPCM format. This would only allow to save some floppy space, since Paula still needs PCM data.

Basically ADPCM is storing 4-bit deltas between samples instead of 8-bit samples, so this means around 50% size decrease without much distortion. Perhaps file format should be able to emit the 8-bit sample if 4-bit delta can't handle lossless transition. Maybe there are some standarised file formats for this already (P61 uses 4bit deltas, I think)