poly: small refactor and unit tests for `SparsePolynomial` #2971

Job	Run time
List directories for parallelizing tests	3s
Check (stable)	14s
Check (nightly)	1m 13s
Check Documentation	59s
Check Style	6s
Check no_std	35s
Test (stable)	9m 19s
Test assembly	8m 17s
Test (nightly)	17m 35s
Test (cp6_782/)	9s
Test (mnt4_753/)	13s
Test (mnt6_753/)	11s
Test (./ed_on_mnt4_753)	3m 57s
Test (./secp256r1)	59s
Test (./ed_on_mnt4_298)	1m 26s
Test (./ed_on_cp6_782)	1m 15s
Test (./mnt4_298)	3m 53s
Test (./mnt6_298)	5m 23s
Test (./ed_on_bn254)	1m 12s
Test (./bn254)	2m 20s
Test (./secp256k1)	1m 12s
Test (./bw6_761)	4m 8s
Test (./bls12_377)	4m 25s
Test (./grumpkin)	1m 17s
Test (./secp384r1)	1m 31s
Test (./ed_on_bls12_377)	1m 17s
Test (./secq256k1)	1m 3s
Test (./ed25519)	57s
Test (./ed_on_bls12_381)	1m 29s
Test (./ed_on_bw6_761)	41s
Test (./curve25519)	1m 2s
Test (./ed_on_bls12_381_bandersnatch)	1m 10s
Test (./pallas)	1m 17s
Test (./vesta)	1m 11s
Test (./bw6_767)	4m 7s
Test (./bls12_381)	3m 24s
	1h 29m 30s

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