GLV parameters structure, v2

[mmagician]

Description

Building on the work from #588 and introducing slight interface & implementation improvements:

1. Use [(bool, Self::ScalarField); 4] instead of separate structs for bools and scalars. (Best would be to use [(bool, num_bigint::BigInt); 4] but const initialization isn't supported for BigInt.
2. Simplify internal impl details by using num_bigint::BigInt instead of BigUint - this way we don't need to keep track of all the signs explicitly.

Benches:

• without GLV:

Arithmetic for Bls12_381::G1/Scalar Multiplication
                        time:   [148.64 µs 148.69 µs 148.73 µs]

• with GLV:

Arithmetic for Bls12_381::G1/Scalar Multiplication
			time:   [94.551 µs 94.621 µs 94.693 µs]

closes: #588

@simonmasson

---

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[Pratyush]

Might be worth it to add endomorphism(_affine)_in_place methods to avoid having to create a copy in implementations.

[mmagician]

I think it needs at least one copy anyway:

algebra/ec/src/scalar_mul/glv.rs

Line 82 in 83b4d6c

let mut b1 = p;

[Pratyush]

@mmagician thank you for the PR! I have left some comments.