Implement single-cell elimination optimization

[str4d]

If I have time, I'll see if I can get another halo2 optimization in that enables these single-cell regions to be optimized away. As long as a single-cell region is used in an equality constraint somewhere, and the column it is assigned into isn't part of a lookup table, the region can just be dropped (and removed from the equality permutation).

Originally posted by @str4d in zcash/orchard#67 (comment)

[str4d]

Replaced by #334 (which also covers multi-cell regions).