BW6-761 part 1

[mratsim]

Part 1 implementation of BW6-761 from https://eprint.iacr.org/2020/351.pdf

The part 1 consists of

• Finite field Fp
• Extension fields Fp2 and Fp6
• G1 curve over Fp
• G2 curve over Fp

Endomorphisms (GLV, Frobenius, lattice decompositions) and pairings are left for subsequent PRs.

BW6-761 necessitated a complete refactor of the Elliptic affine/projective/jacobian type.
I had to add a new OnTwist dependent type/static enum as:

1. BW6-761 has an embedding degree of 6
2. BW6-761 has a sextic twist
3. The Twisted curve is over Fp^(embedding_degree/twist_degree) = Fp^(6/6) = Fp hence both G1 and G2 are defined on Fp (unlike BN and BLS12 where G1 is on Fp and G2 on Fp2).

so we can't assume G1 or G2 from just the base field.