Refactor prove + verify to use helper functions

[jdwhite48]

Related: #20

• Currently, there is a lot of repeated code in prove that could be shared:

• PPE and MSMEG2 have functionally identical computations for π, except that the commitment randomness for x, i.e. r, is a 1-column vector instead of a 2-column matrix R and the size of used randomness T is different. Call this e.g. prove_vars_in_G2.

• PPE and MSMEG1 have functionally identical computations for θ, except that the commitment randomness for y, i.e. s, is a 1-column vector instead of a 2-column matrix S and the size of used randomness T is different. Call this e.g. prove_vars_in_G1.

• MSMEG1 and QuadEqu have functionally identical computations for π, except that r is a 1-column vector instead of a 2-column and the size of T is different. Call this e.g. prove_scalar_vars_for_B2.

• MSMEG2 and QuadEqu have functionally identical computations for θ, except that s is a 1-column vector instead of a 2-column and the size of T is different. Call this e.g. prove_scalar_vars_for_B1.

• Similarly there is a lot of repeated code in verify that could be shared (although slightly less so):

• PPE and MSMEG1 have functionally identical 1st and 3rd LHS terms (only differing in where variables originated before being committed) and identical 2nd RHS term. Call this private helper function e.g. verify_G1_vars.

• PPE and MSMEG2 have functionally identical 2nd LHS term and identical 3rd RHS term. Call this e.g. verify_G2_vars.

• MSMEG1 and QuadEqu have functionally identical 2nd LHS term and identical 3rd RHS term. Call this e.g. verify_scalar_consts_from_B2.

• MSMEG2 and QuadEqu have functionally identical 1st and 3rd LHS terms and identical 2nd RHS term. Call this e.g. verify_scalar_consts_from_B1.