Fix panic bugs in ark_poly Add and Sub impls

[teor2345]

Some missing truncate_leading_zeroes() in ark_poly operator impls can cause a panic in degree():

• impl<'a, 'b, F: Field> Add<&'a SparsePolynomial> for &'b DensePolynomial at https://github.com/arkworks-rs/algebra/blob/97c7a4f4e8f5e297e397f6de09fe4764068deac0/poly/src/polynomial/univariate/dense.rs#L330
• impl<'a, F: Field> AddAssign<&'a SparsePolynomial> for DensePolynomial at https://github.com/arkworks-rs/algebra/blob/97c7a4f4e8f5e297e397f6de09fe4764068deac0/poly/src/polynomial/univariate/dense.rs#L414
• impl<'a, 'b, F: Field> Sub<&'a SparsePolynomial> for &'b DensePolynomial at https://github.com/arkworks-rs/algebra/blob/97c7a4f4e8f5e297e397f6de09fe4764068deac0/poly/src/polynomial/univariate/dense.rs#L490
• impl<'a, F: Field> SubAssign<&'a SparsePolynomial> for DensePolynomial at https://github.com/arkworks-rs/algebra/blob/97c7a4f4e8f5e297e397f6de09fe4764068deac0/poly/src/polynomial/univariate/dense.rs#L546

We can submit a PR to ark_poly, get it merged, and then upgrade to the next release.

In the meantime, we can add a truncate_to_canonical_form() in our own Poly impls. This means we can't use derive_more macros, unless the underlying implementation is correct.

[teor2345]

Currently this is fixed by depending on arkworks-rs/algebra#638 via arkworks-rs/algebra@8233e9a in PR #32.

We don't use SparsePolynomial, so this bug shouldn't happen to us.