poly: small refactor in `SparseTerm` `partial_cmp` #2990

Job	Run time
List directories for parallelizing tests	4s
Check (stable)	11s
Check no_std	27s
Check (nightly)	1m 18s
Check Documentation	1m 2s
Check Style	5s
Test (stable)	9m 1s
Test assembly	8m 17s
Test (nightly)	17m 24s
Test (cp6_782/)	8s
Test (mnt4_753/)	16s
Test (mnt6_753/)	10s
Test (./ed_on_mnt4_753)	3m 48s
Test (./secp256r1)	1m 12s
Test (./ed_on_mnt4_298)	1m 8s
Test (./ed_on_cp6_782)	1m 14s
Test (./mnt4_298)	3m 43s
Test (./mnt6_298)	5m 32s
Test (./ed_on_bn254)	59s
Test (./bn254)	2m 19s
Test (./secp256k1)	1m 2s
Test (./bw6_761)	3m 57s
Test (./bls12_377)	4m 32s
Test (./grumpkin)	1m 8s
Test (./secp384r1)	1m 28s
Test (./ed_on_bls12_377)	1m 5s
Test (./secq256k1)	1m 2s
Test (./ed25519)	53s
Test (./ed_on_bls12_381)	1m 10s
Test (./ed_on_bw6_761)	38s
Test (./curve25519)	1m 2s
Test (./ed_on_bls12_381_bandersnatch)	1m 5s
Test (./pallas)	1m 2s
Test (./vesta)	1m 9s
Test (./bw6_767)	3m 59s
Test (./bls12_381)	3m 16s
	1h 26m 46s

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