Complete adding mix and amp args to all FX -

also some tweaks and mods:

* compressor now uses a mix of left and right chans as the level sig so
  both left and right chans are compressed identically (rather than
  having independent compressors)
* compressor now supports a pre_amp arg to allow input signal to be amped
  prior to compression
* normalised lpf synthinfo now inherits from lpf
* bitcrusher defaults now do some bitcrushing!