Pass a maximum lookback of n to the circle-breaking DP of size 2*n.
This should theoretically speed up that DP by a factor of 4 or 8 or something...
Empirically it's far from that, but solidly measurable.
Again, gvar table size got a slight improvement, which I didn't expect... It
might be that my one test is hitting lucky cases with point encodings.. or it
might be just me wishfully thinking so.