Use linear combinations for rowcheck

[Pratyush]

To optimize the NARK and the accumulation verifier, we could use the "open a linear-equation" idea from Marlin, where for row-check instead of providing evaluations for z_a, z_b, z_c, v_h and q, we provide evaluations for z_a and v_H only, and then check that z_a(beta) * z_b - z_c - v_H(beta) * q = 0 at beta

this reduces the accumulator size, and eliminates some non-native work

(above I assume that we're committing to v_H too)