Add Field::sum_of_products method

[str4d]

Closes #79.

[str4d]

Force-pushed to remove the ff_derive efficient override (which I don't have time to prove correct), and change the API to consume an iterator of pairs of references instead of two arrays (to make it more flexible).

[str4d]

I originally made this an iterator of &(a, b), and I tested this against the array impl and saw little-to-no performance difference. I then changed it to an iterator of (&a, &b) because I thought it would be more flexible, but doing that significantly hurts performance (I presume because we're calling .clone() on an owned tuple that contains references, rather than on a reference).

[str4d]

Yeah, even using nicer tricks for constructing the input arrays, having this API take (&a, &b) causes bls12_381 pairings to be over 5% slower compared to arrays or &(a, b).

[str4d]

Tried two iterators instead:

fn sum_of_products<'a>(
        a: impl IntoIterator<Item = &'a Self> + Clone,
        b: impl IntoIterator<Item = &'a Self> + Clone,
    ) -> Self

and then a.clone().into_iter().zip(b.clone().into_iter()); this is similarly slower than arrays.

[str4d]

Rebased on current main.

[str4d]

Force-pushed to re-introduce the faster array-based API (moving the iterator-based API to a different name).

[str4d]

Rebased on current main.

While this PR adds APIs that could be introduced in a patch release, I'll instead propose adding them in 0.14.0.

[str4d]

Force-pushed to fix rebase conflicts.