Support 2 SNARK configuration flags

[AntoineRondelet]

Nothing prevents the nested and wrapping SNARKs to be different (see: #16 (comment)). Let's make the configuration generic to support that.

[dtebbs]

Just a note regarding this... On reflection, I think this may require a slight reorganization of types afterall.
I used nsnarkT<nppT> and wverifier<wppT> where wsnarkT = wverifier<wppT>::snark when originally defining these types, but I think this will need changing so that wverifier<wppT> (which determines the set of gadgets and variable to be used) is completely independent of wsnarkT, matching only nsnarkT.

That is, for wrapping curve wppT, groth16_verifier<wppT> (say) defines the gadgets, key variable types, proof variable types, etc for verifying groth16 nested proofs. This then fixes nsnark = groth16_verifier<wppT>::snark = groth16_snark<other_curve<wppT>> = groth16_snark<nppT>. wsnarkT determines the concrete types used for generating and verifying the outer proofs, so should be set independently of the inner verifier groth16_verifier<wppT>.