poly: small refactor for `SparsePolynomial` #3017

Job	Run time
Test (stable)	8m 53s
Check (stable)	8s
Test (nightly)	17m 29s
Check (nightly)	1m 18s
Check no_std	24s
List directories for parallelizing tests	3s
Check Documentation	1m 7s
Check Style	21s
Test assembly	8m 27s
Test (cp6_782/)	4s
Test (mnt4_753/)	11s
Test (mnt6_753/)	10s
Test (./ed_on_mnt4_753)	3m 46s
Test (./secp256r1)	55s
Test (./ed_on_mnt4_298)	1m 8s
Test (./ed_on_cp6_782)	1m 26s
Test (./mnt4_298)	3m 41s
Test (./mnt6_298)	5m 23s
Test (./ed_on_bn254)	57s
Test (./bn254)	2m 22s
Test (./secp256k1)	1m 10s
Test (./bw6_761)	4m 6s
Test (./bls12_377)	4m 6s
Test (./grumpkin)	1m 6s
Test (./secp384r1)	1m 19s
Test (./ed_on_bls12_377)	1m 17s
Test (./secq256k1)	53s
Test (./ed25519)	1m 0s
Test (./ed_on_bls12_381)	1m 15s
Test (./ed_on_bw6_761)	39s
Test (./curve25519)	1m 3s
Test (./ed_on_bls12_381_bandersnatch)	1m 3s
Test (./pallas)	1m 17s
Test (./vesta)	1m 10s
Test (./bw6_767)	3s
Test (./bls12_381)	3m 15s
	1h 22m 55s

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