poly: small refactor and unit tests for `SparsePolynomial` (#868) #2979

Job	Run time
Check no_std	32s
Check (stable)	7s
Check (nightly)	1m 22s
List directories for parallelizing tests	3s
Check Documentation	58s
Check Style	10s
Test (stable)	8m 58s
Test assembly	8m 32s
Test (nightly)	17m 33s
Test (cp6_782/)	7s
Test (mnt4_753/)	17s
Test (mnt6_753/)	12s
Test (./ed_on_mnt4_753)	3m 56s
Test (./secp256r1)	58s
Test (./ed_on_mnt4_298)	1m 18s
Test (./ed_on_cp6_782)	1m 17s
Test (./mnt4_298)	4m 0s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	58s
Test (./bn254)	2m 14s
Test (./secp256k1)	1m 8s
Test (./bw6_761)	3m 52s
Test (./bls12_377)	4m 4s
Test (./grumpkin)	1m 4s
Test (./secp384r1)	1m 31s
Test (./ed_on_bls12_377)	1m 8s
Test (./secq256k1)	1m 0s
Test (./ed25519)	56s
Test (./ed_on_bls12_381)	1m 15s
Test (./ed_on_bw6_761)	39s
Test (./curve25519)	1m 5s
Test (./ed_on_bls12_381_bandersnatch)	59s
Test (./pallas)	1m 15s
Test (./vesta)	1m 5s
Test (./bw6_767)	4m 12s
Test (./bls12_381)	3m 17s
	1h 27m 28s

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