poly: unit tests and refac for `DensePolynomial` addition mixed #2987

Job	Run time
Check (stable)	13s
Check no_std	31s
Check (nightly)	1m 10s
List directories for parallelizing tests	3s
Check Documentation	54s
Check Style	8s
Test (stable)	8m 52s
Test (nightly)	17m 34s
Test assembly	8m 23s
Test (cp6_782/)	9s
Test (mnt4_753/)	13s
Test (mnt6_753/)	12s
Test (./ed_on_mnt4_753)	3m 47s
Test (./secp256r1)	53s
Test (./ed_on_mnt4_298)	1m 18s
Test (./ed_on_cp6_782)	1m 15s
Test (./mnt4_298)	3m 46s
Test (./mnt6_298)	5m 23s
Test (./ed_on_bn254)	1m 3s
Test (./bn254)	2m 19s
Test (./secp256k1)	1m 18s
Test (./bw6_761)	3m 50s
Test (./bls12_377)	4m 7s
Test (./grumpkin)	1m 5s
Test (./secp384r1)	1m 17s
Test (./ed_on_bls12_377)	1m 8s
Test (./secq256k1)	58s
Test (./ed25519)	55s
Test (./ed_on_bls12_381)	1m 10s
Test (./ed_on_bw6_761)	36s
Test (./curve25519)	1m 3s
Test (./ed_on_bls12_381_bandersnatch)	1m 1s
Test (./pallas)	1m 11s
Test (./vesta)	1m 12s
Test (./bw6_767)	4m 3s
Test (./bls12_381)	3m 26s
	1h 26m 26s

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