Optimize GS bilinear group arithmetic

[jdwhite48]

Review all of the uses of PairingEngine::pairing, ComT::pairing, and bilinear group arithmetic:

• Store all group elements in the using the AffineCurve representation, to save on the size of group elements
• Only perform pairing operations if neither of the operands are zero; just return Fqk::one() or ComT::zero() otherwise (I don't know if Arkworks optimizes for this already)
• Use the multiplication function in ProjectiveCurve instead, if possible / more efficient than the one currently used in AffineCurve
• Use the "pre-batching" techniques in [HHK '17] to reduce the number of pairings necessary to verify Groth-Sahai equations