Avoid initialization

[recmo]

On 2019-09-11 @recmo wrote in 7ced6e0 “Coset parallel LDE”:

Avoid initialization

        let generator =
            FieldElement::root(length).expect("No generator for extended_domain_length.");
        let mut result: MmapVec<FieldElement> = MmapVec::with_capacity(length);

        // Initialize to zero
        // TODO: Avoid initialization
        result.resize(length, FieldElement::ZERO);

        // Compute cosets in parallel
        result
            .as_mut_slice()

From crypto/openstark/src/polynomial.rs:83