Reduce number of stored group elements for proofs about linear equations

[jdwhite48]

The original Groth-Sahai paper [GS '08] suggests that it is possible to reduce the number of group elements if the equations have a linear format.

• Detect if the Equation provided as input is linear in form: at one of every pair of elements in A,Y (sim., X,B) is zero for each type of equation, and Γ is the zeroes matrix).
• If the equation is linear, one of the proofs will have all of its group elements be zero; refactor π and θ to have type Option<Vec<_>>, where the None constructor is trivially interpreted as the zero vector.