Remove SquareRootField, and move functionality to Field

[mmagician]

Description

Rather than implementing optimised algorithms for each case from the start, let's begin with Tonelli-Shanks, which should work in each instance, then benchmark that and continue to implement optimisations for each case one-by-one. While not fastest from the start, it already unifies the SqrtField and Field traits and makes using of sqrt() method cleaner.

In this draft, I propose moving the following to CubicExtField (prev. only on Fp3):

• TWO_ADICITY
• TRACE_MINUS_ONE_DIV_TWO
• QUADRATIC_NONRESIDUE_TO_T

This puts a small overhead on implementers of CubicExtField which are not Fp3 (e.g. Fq6), requiring these to define the above constants.

TODOs:

• verify the approach (based on feedback)
• implement the above constants for test-curves
• implement the above for all curves
• (?) generate the constants like done here

Credit to @solomonjoseph for the initial work on this.

closes: #387

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[Pratyush]

Why the reason for this change?

[mmagician]

The calling method, sqrt, was previously on a SquareRootField trait, and in the implementation for quadratic extension we were restricting P: QuadExtConfig to have:

where P::BaseField: From<P::BasePrimeField>

Now that the method is on Field trait, we don't have these bounds, so I need to explicitly construct a base field element from BasePrimeField elements.

[Pratyush]

Let's add a BaseField: From<BasePrimeField> bound in the config struct; maybe as a different PR?

[Pratyush]

I had discussed this with Solomon in the past, but a good way to unify the preprocessing for all the various square root algorithms would be to make an enum of the form

pub enum SqrtPreprocessing<F: Field> {
    OneMod16(...),
    NineMod16(...),
    ThreeMod4(...),
    // and more...
}

And then, in Field, we add a constant for this enum:

pub trait Field {
    const SQRT_PREPROCESSING: Option<SqrtPreprocessing<Self>>;
}

The Option is there to allow certain implementations to say unimplemented when Sqrts are not used there (but I'm flexible on this point).

[mmagician]

For now I've added only a SqrtPrecomputation::TonelliShanks variant, which should work for all fields. Then we can start implementing specialised algorithms for other variants such as OneMod16 etc.