Add allocator for DensePolynomialVar

[tsunrise]

Summary

Add allocator for DensePolynomialVar, so we can easily convert ark_poly::DensePolynomial to DensePolynomialVar by calling DensePolynomial::new_variable.

We can keep release 3.0 as is. It's ok to move this feature to the next release.

Proposal

Impl AllocVar for DensePolynomial.

I also plan to impl AllocVar for domains and evaluations so that we can easily convert univariate domain in ark-poly to variable. However, currently univariate domain can only be subgroup (instead of coset). Once coset is supported, we can so do.

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