Address broken support for folding multiple instances

[ledwards2225]

We don't support folding multiple instances but there are places in the code where fragments of deprecated support still exist. Aside from making the code more complex there may actually be performance hits. Zac points out one such case where in polynomial folding we do

acc[i] = acc[i] * lagranges[0] + key[i] * lagranges[1]

instead of the equivalent

acc[i] = acc[i] + (key[i] - acc[i]) * gamma

which saves a mul per row per poly. This could be significant given the somewhat inefficient multithreading strategy for poly folding.