poly: small refactor for `SparsePolynomial` #3016

Job	Run time
Check Style	8s
List directories for parallelizing tests	3s
Check (stable)	7s
Check Documentation	1m 0s
Check (nightly)	1m 13s
Test (stable)	8m 53s
Test (mnt4_753/)	12s
Test (nightly)	17m 36s
Test (mnt6_753/)	9s
Check no_std	24s
Test assembly	8m 18s
Test (cp6_782/)	12s
Test (./ed_on_mnt4_753)	3m 47s
Test (./secp256r1)	53s
Test (./ed_on_mnt4_298)	1m 7s
Test (./ed_on_cp6_782)	1m 22s
Test (./mnt4_298)	3m 55s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	57s
Test (./bn254)	2m 18s
Test (./secp256k1)	1m 5s
Test (./bw6_761)	3m 56s
Test (./bls12_377)	4m 12s
Test (./grumpkin)	1m 4s
Test (./secp384r1)	1m 19s
Test (./ed_on_bls12_377)	1m 12s
Test (./secq256k1)	57s
Test (./ed25519)	58s
Test (./ed_on_bls12_381)	1m 10s
Test (./ed_on_bw6_761)	39s
Test (./curve25519)	1m 1s
Test (./ed_on_bls12_381_bandersnatch)	1m 0s
Test (./pallas)	1m 3s
Test (./vesta)	1m 7s
Test (./bw6_767)	3m 59s
Test (./bls12_381)	3m 20s
	1h 26m 2s

Provide feedback