Make PoK works for both G1 and G2

[drskalman]

We should maybe make this PoK do both G1 and G2 if we're going that route, no? I guess that's a seperate thing that depends upon only Engine.

[burdges]

It'll be a Schnorr proof on both curves as once, so we'll likely define it over the underlying Engine, not over EngineBLS

[burdges]

We should merge features of this repo into and deprecate this repo for nugget_bls which does this correctly.