poly: small refactor and unit tests for `SparsePolynomial` #2972

Job	Run time
Check (stable)	13s
Check Documentation	1m 3s
Check (nightly)	1m 13s
Check no_std	27s
List directories for parallelizing tests	3s
Check Style	10s
Test (stable)	8m 51s
Test (nightly)	17m 29s
Test assembly	8m 24s
Test (cp6_782/)	11s
Test (mnt4_753/)	13s
Test (mnt6_753/)	9s
Test (./ed_on_mnt4_753)	4m 35s
Test (./secp256r1)	1m 4s
Test (./ed_on_mnt4_298)	1m 13s
Test (./ed_on_cp6_782)	1m 27s
Test (./mnt4_298)	3m 58s
Test (./mnt6_298)	5m 25s
Test (./ed_on_bn254)	1m 4s
Test (./bn254)	2m 31s
Test (./secp256k1)	1m 7s
Test (./bw6_761)	3m 54s
Test (./bls12_377)	4m 7s
Test (./grumpkin)	1m 12s
Test (./secp384r1)	1m 20s
Test (./ed_on_bls12_377)	1m 2s
Test (./secq256k1)	1m 2s
Test (./ed25519)	1m 6s
Test (./ed_on_bls12_381)	1m 17s
Test (./ed_on_bw6_761)	47s
Test (./curve25519)	1m 12s
Test (./ed_on_bls12_381_bandersnatch)	1m 13s
Test (./pallas)	1m 18s
Test (./vesta)	1m 51s
Test (./bw6_767)	4m 5s
Test (./bls12_381)	3m 19s
	1h 29m 35s

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