Prove about multiple Groth-Sahai equations simultaneously

[jdwhite48]

Constructing individual proofs for each individual GS equation is tedious when the proof system is easily designed to prove about the satisfiability of multiple equations simultaneously

• Implement the appending of committed variables
• Provide helper functions for composing existing Groth-Sahai proofs together; refactor to Vec<EquProof> and naïvely compose (with potential duplication of variables) by adding zero elements corresponding to other equations' variables
• Implement prove and verify functions that are defined over the entire Statement (i.e. Vec of Equation)
• Test that individually proving about each equation and proving about them all together give equivalent proofs (not necessarily identical Equation constructors)

[jdwhite48]

More TODOs:

• Generalize Statement to conform to an arbitrary Groth-Sahai implementation of Equation
• Have Commit1 and Commit2 include Option<_> for x vars in G1Affine, y vars in G2Affine, and x, y vars in Fr