SQUASH: fate-opus: silk shows float instability,

use stddev test for any sample using the silk decoder
add fate-opus-celt/hybrid/silk for tests which use one mode exclusively
Residuals do not need to be stored across frames.
opus: remove clipping

Second equation of 4.2.7.9.1. LTP Synthesis does not have clipping.
opus: correct usage of LTP order

Old code had a mixup of equations. Using the LTP order makes it
clearer what the loops should be. No actual change to output.
opus: prefer rescaling

The astute reader will notice that, given the definition of
lpc[i] in Section 4.2.7.9.2, the output of this latter equation is
merely a scaled version of the values of res[i] from previous
subframes.
opus: use an accumulator

This does change the results for LPC synthesis, but within tolerance.