refactor: clean up 2-chain gadgets and expose only algebra.Curve and algebra.Pairing-compatible methods

[ivokub]

I'll leave cleaning up the native EC gadgets as a separate PR. I think we can have other discussions there i.e. we can try to implement Jacobian operations such that it also implements Curve interface etc.

Originally posted by @ivokub in #840 (comment)

• perf: direct MillerLoop in 2-chains precomputes G2 lines on copies #963 963

And I guess we can also clean up emulated packages. During development we added methods and fields which are not used anymore (addStep, generators() etc.). See #949