poly: add specific unit tests and simplify `DensePolynomial` addition #2975

Job	Run time
Check Documentation	52s
Check (stable)	15s
Check (nightly)	1m 18s
Check Style	13s
Check no_std	46s
List directories for parallelizing tests	2s
Test (stable)	8m 59s
Test (nightly)	17m 33s
Test assembly	8m 20s
Test (cp6_782/)	15s
Test (mnt4_753/)	11s
Test (mnt6_753/)	11s
Test (./ed_on_mnt4_753)	3m 58s
Test (./secp256r1)	1m 9s
Test (./ed_on_mnt4_298)	1m 15s
Test (./ed_on_cp6_782)	1m 17s
Test (./mnt4_298)	4m 2s
Test (./mnt6_298)	5m 24s
Test (./ed_on_bn254)	1m 1s
Test (./bn254)	2m 25s
Test (./secp256k1)	1m 7s
Test (./bw6_761)	4m 2s
Test (./bls12_377)	4m 9s
Test (./grumpkin)	1m 17s
Test (./secp384r1)	1m 20s
Test (./ed_on_bls12_377)	1m 25s
Test (./secq256k1)	1m 9s
Test (./ed25519)	58s
Test (./ed_on_bls12_381)	1m 15s
Test (./ed_on_bw6_761)	54s
Test (./curve25519)	1m 11s
Test (./ed_on_bls12_381_bandersnatch)	1m 14s
Test (./pallas)	1m 5s
Test (./vesta)	1m 3s
Test (./bw6_767)	3m 55s
Test (./bls12_381)	3m 28s
	1h 28m 58s

Provide feedback