poly: unit tests and refac for `DensePolynomial` addition mixed (#870) #3064

Job	Run time
Check Documentation	53s
Check Style	5s
List directories for parallelizing tests	4s
Check no_std	28s
Check (stable)	9s
Check (nightly)	1m 14s
Test assembly	8m 26s
Test (stable)	8m 59s
Test (nightly)	17m 27s
Test (cp6_782/)	6s
Test (mnt4_753/)	13s
Test (mnt6_753/)	14s
Test (./ed_on_mnt4_753)	3m 52s
Test (./secp256r1)	1m 7s
Test (./ed_on_mnt4_298)	1m 5s
Test (./ed_on_cp6_782)	1m 16s
Test (./mnt4_298)	3m 47s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	59s
Test (./bn254)	2m 17s
Test (./secp256k1)	1m 1s
Test (./bw6_761)	3m 59s
Test (./bls12_377)	4m 6s
Test (./grumpkin)	1m 7s
Test (./secp384r1)	1m 17s
Test (./ed_on_bls12_377)	1m 2s
Test (./secq256k1)	54s
Test (./ed25519)	58s
Test (./ed_on_bls12_381)	1m 14s
Test (./ed_on_bw6_761)	37s
Test (./curve25519)	1m 13s
Test (./ed_on_bls12_381_bandersnatch)	1m 1s
Test (./pallas)	1m 5s
Test (./vesta)	1m 5s
Test (./bw6_767)	4m 4s
Test (./bls12_381)	3m 17s
	1h 26m 7s

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