veridise audit fix: redc field is manipulable

[jp4g]

Addresses Veredise Audit Issue: redc field is manipulable

The recommendation was to constrain that redc = floor((1 << (2 * Params::modulus_bits())) / modulus). However, this would greatly negatively impact the efficiency of the circuit so we want to avoid this measure. Since the ultimate goal is to constrain the outputted dkim_key_hash, we bind the redc to the modulus by hashing the two together. The verifier will still need to do this work themselves (only once) to get the expected dkim_key_hash, but it is far more efficient this way and ultimately addresses the core vulnerability.

Since this adds a couple thousand constraints of work we also compose the limbs of the modulus and redcs from two 120 bit numbers into one 240 bit number to negate this increase. In order to do this we had to move pubkey hashing into the pubkey struct since there are different impls for even and odd (1024/2048 bit) limb lengths

Stack order: 2