More PVQ cleanup.

Folds quality scaling directly inside od_gain_compand() and od_gain_expand().
Not bit-exact, but equivalent.
More simplifications to pvq_synthesis_partial().

Again, not bit-exact, but equivalent.
Moves pvq.c functions to encoder/decoder-specific files

No change to the functions themselves
fixes some comments
Jointly signalling whether each of the directions should be skipped

subset1:
           RATE (%) DSNR (dB)
    PSNR -0.0609499 0.00364128
 PSNRHVS -0.0845392 0.00576046
    SSIM -0.0620935 0.00300164
FASTSSIM -0.0871825 0.00281497

video1-short:
           RATE (%)  DSNR (dB)
    PSNR -0.02604  0.00133
 PSNRHVS -0.02885  0.00214
    SSIM -0.00334  0.00015
FASTSSIM 0.02953  -0.00168