Add alloc feature flag

[str4d]

In #4 we introduced a std feature flag, to enable downstream users to disable anything relying on std. This happened to gate all of the traits, because both FieldExt and CurveAffine depended on std::io.

We removed std::io usage from FieldExt in #20, so it could now be removed from any feature flag. CurveAffine still relies on std::io, but I want to remove that as well. Then we're just left with the fact that CurveExt requires FieldExt, FieldExt is bound on SqrtRatio, and the only way we can implement SqrtRatio right now depends on SqrtTables, which requires alloc.

So, we should add an alloc feature flag, and move everything behind it. We might also be able to remove the std feature flag as a result (which has not been in a release yet). There's no point in having the traits themselves be no-std accessible if we can't implement them without alloc. Once we have a fallback for SqrtRatio we can consider making the other traits no-std usable.

[krnak]

Another reason, why CurveExt cannot be accessible without alloc, is that hash_to_curve still depends on Box.

However I would like to be able to compile the crate without sqrt tables to safe memory of our tiny device. I remind you that these tables are only optimization. We already can calculate square roots via Tonelli-Shanks algorithm without tables. SqrtRatio can be implemented generically as

fn sqrt_ratio(num: &Self, div: &Self) -> (Choice, Self) {
    //generic implementation:

    //invert div
    let div_inv = (*div).invert().unwrap_or(Self::zero());

    let x = (*num) * div_inv; // x = num/div
    let y = x * Self::root_of_unity(); // y = lambda*num/div

    let x_sqrt = x.sqrt();
    let y_sqrt = y.sqrt();

    // Either x is square or y is square
    let normal_sqrt = CtOption::conditional_select(&y_sqrt, &x_sqrt, x_sqrt.is_some());
    let special_sqrt = CtOption::new(Self::zero(), 1.into());
    let result_sqrt = CtOption::conditional_select(&normal_sqrt, &special_sqrt, div.is_zero());

    let normal_bool = x_sqrt.is_some();
    let special_bool = !div.is_zero();
    let result_bool = Choice::conditional_select(&normal_bool, &special_bool, div.is_zero());

    (result_bool, result_sqrt.unwrap())
}

or a little bit more efficiently by separating first part of Tonelli-Shanks algorithm (where the squareness of an input is checked).

May we create a new feature flag to conditionally disable these tables?

[str4d]

Hmm, that's a good point about separating the table impl from alloc. Yeah, I think it would be fine to have a sqrt-table flag that depends on alloc to provide them. We can workshop the API beyond that later.