refactor `sub_assign` `DensePolynomial` #3121

Job	Run time
Check Style	6s
Check (stable)	59s
Check (nightly)	1m 3s
Check no_std	27s
List directories for parallelizing tests	3s
Check Documentation	55s
Test (stable)	8m 56s
Test assembly	8m 18s
Test (cp6_782/)	5s
Test (nightly)	17m 23s
Test (mnt4_753/)	12m 58s
Test (mnt6_753/)	24m 50s
Test (./ed_on_mnt4_753)	3m 54s
Test (./secp256r1)	57s
Test (./ed_on_mnt4_298)	1m 3s
Test (./ed_on_cp6_782)	1m 18s
Test (./mnt4_298)	3m 44s
Test (./mnt6_298)	5m 30s
Test (./ed_on_bn254)	59s
Test (./bn254)	2m 19s
Test (./secp256k1)	1m 2s
Test (./bw6_761)	3m 51s
Test (./bls12_377)	4m 19s
Test (./grumpkin)	1m 14s
Test (./secp384r1)	1m 24s
Test (./ed_on_bls12_377)	1m 4s
Test (./secq256k1)	55s
Test (./ed25519)	55s
Test (./ed_on_bls12_381)	1m 12s
Test (./ed_on_bw6_761)	37s
Test (./curve25519)	1m 3s
Test (./ed_on_bls12_381_bandersnatch)	1m 5s
Test (./pallas)	1m 4s
Test (./vesta)	1m 5s
Test (./bw6_767)	4m 2s
Test (./bls12_381)	3m 11s
	2h 3m 50s

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