`DensePolynomial`: add `to_coefficients_vec` #3128

Job	Run time
Check Documentation	55s
Check no_std	24s
List directories for parallelizing tests	3s
Check (stable)	1m 6s
Check (nightly)	1m 3s
Check Style	6s
Test (stable)	9m 1s
Test assembly	8m 18s
Test (nightly)	17m 35s
Test (cp6_782/)	28m 54s
Test (mnt4_753/)	10m 45s
Test (mnt6_753/)	30m 55s
Test (./pallas)	1m 11s
Test (./ed25519)	55s
Test (./ed_on_mnt4_753)	3m 49s
Test (./bn254)	2m 22s
Test (./bls12_381)	3m 12s
Test (./secp384r1)	1m 19s
Test (./bw6_767)	3m 51s
Test (./grumpkin)	1m 19s
Test (./curve25519)	1m 3s
Test (./ed_on_bls12_377)	1m 4s
Test (./ed_on_mnt4_298)	1m 15s
Test (./vesta)	1m 10s
Test (./ed_on_bls12_381_bandersnatch)	1m 13s
Test (./bls12_377)	4m 9s
Test (./ed_on_bw6_761)	39s
Test (./mnt4_298)	3m 48s
Test (./secp256k1)	1m 4s
Test (./ed_on_bn254)	1m 2s
Test (./ed_on_bls12_381)	1m 17s
Test (./secp256r1)	57s
Test (./bw6_761)	3m 54s
Test (./secq256k1)	56s
Test (./mnt6_298)	5m 22s
Test (./ed_on_cp6_782)	1m 16s
	2h 37m 12s

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