Refactor BN curves.

[davidnevadoc]

The BN curves Pluto and bn256 can a good amount of code that is currently duplicated (once #98 is merged). In order to fix this we should introduce macros for:

• Field extensions.
• BN pairing.

This will be a good opportunity to refactor some of bn256 code which is in many parts undocumente and at times quite unclear.

[mratsim]

Ideally I would also do renaming/breaking changes:

• rename to bn254
• Engine is a very bad name for the pairing trait and it should be called Pairing:

  halo2curves/src/bn256/engine.rs

  Lines 635 to 646 in 2e7f8eb

  	impl Engine for Bn256 {
  	type Fr = Fr;
  	type G1 = G1;
  	type G1Affine = G1Affine;
  	type G2 = G2;
  	type G2Affine = G2Affine;
  	type Gt = Gt;
  	fn pairing(p: &Self::G1Affine, q: &Self::G2Affine) -> Self::Gt {
  	pairing(p, q)
  	}
  	}