refactor `DensePolynomial` add #3115

Job	Run time
Check no_std	25s
Check (stable)	1m 5s
List directories for parallelizing tests	5s
Check (nightly)	1m 3s
Check Documentation	52s
Check Style	6s
Test (stable)	8m 57s
Test assembly	8m 17s
Test (nightly)	17m 40s
Test (cp6_782/)	28m 43s
Test (mnt4_753/)	13m 40s
Test (mnt6_753/)	26m 12s
Test (./ed_on_mnt4_753)	3m 49s
Test (./secp256r1)	59s
Test (./ed_on_mnt4_298)	1m 4s
Test (./ed_on_cp6_782)	1m 32s
Test (./mnt4_298)	3m 44s
Test (./mnt6_298)	5m 23s
Test (./ed_on_bn254)	1m 1s
Test (./bn254)	2m 14s
Test (./secp256k1)	1m 15s
Test (./bw6_761)	3m 51s
Test (./bls12_377)	4m 8s
Test (./grumpkin)	1m 7s
Test (./secp384r1)	1m 21s
Test (./ed_on_bls12_377)	1m 7s
Test (./secq256k1)	57s
Test (./ed25519)	56s
Test (./ed_on_bls12_381)	1m 18s
Test (./ed_on_bw6_761)	38s
Test (./curve25519)	1m 7s
Test (./ed_on_bls12_381_bandersnatch)	1m 12s
Test (./pallas)	1m 14s
Test (./vesta)	1m 4s
Test (./bw6_767)	4m 1s
Test (./bls12_381)	3m 13s
	2h 35m 20s

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