Effect interface update This updates the effects to the form my microcontroller project is currently using: - move the effects headers into a 'audio' subdirectory - use a more complete 'struct effect' model with pot descriptors - updated and extended math helper routines This updates a few core effects, brings a few others in from the microcontroller code (boost / pitch), and removes some others that were either pointless (fm/am signal generation) or didn't sound great or weren't very educational (pll / growlingbass / distortion / tube). Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>