Refactor API to allow other instantiations

[jdwhite48]

While many regard the most efficient instantiation for Groth-Sahai to be under the SXDH instantiation (and using Type-III setting in general), it might be good to restructure the code to allow for other instantiations in the literature such as DLIN in the Type-I pairing setting [GS '08] or SDLIN extended to the Type-II pairing setting [GSW '10].

While I have had this in mind from the start, this still would require quite a bit of refactoring:

• Figure out if Arkworks even supports curves besides Type-III pairing setting
• Migrate all existing code into a folder that indicates that it is the SXDH instantiation
• Generalize CRS to just have the commitment key vectors (u, v), with choice of generators left up to instantiation
• Pull out the abstracted versions of Groth-Sahai components -- bilinear group definitions (B1, B2, BT), Equation, Statement, GSType, EquProof, etc.
• Figure out how to generalize the 4 kinds of equations -- PPE, MSMEG1, MSMEG2, Quadratic
• Introduce more traits as needed to generalize the Groth-Sahai construction further