poly: small refac for `GeneralEvaluationDomain` #2991

Job	Run time
List directories for parallelizing tests	3s
Check (stable)	10s
Check no_std	22s
Check (nightly)	1m 9s
Check Documentation	53s
Check Style	7s
Test (stable)	9m 1s
Test assembly	8m 23s
Test (cp6_782/)	9s
Test (nightly)	17m 29s
Test (mnt4_753/)	12s
Test (mnt6_753/)	11s
Test (./ed_on_mnt4_753)	4m 5s
Test (./secp256r1)	57s
Test (./ed_on_mnt4_298)	1m 5s
Test (./ed_on_cp6_782)	1m 12s
Test (./mnt4_298)	3m 57s
Test (./mnt6_298)	5m 27s
Test (./ed_on_bn254)	56s
Test (./bn254)	2m 16s
Test (./secp256k1)	1m 4s
Test (./bw6_761)	3m 55s
Test (./bls12_377)	4m 14s
Test (./grumpkin)	1m 15s
Test (./secp384r1)	1m 18s
Test (./ed_on_bls12_377)	1m 7s
Test (./secq256k1)	58s
Test (./ed25519)	54s
Test (./ed_on_bls12_381)	1m 16s
Test (./ed_on_bw6_761)	46s
Test (./curve25519)	1m 3s
Test (./ed_on_bls12_381_bandersnatch)	1m 1s
Test (./pallas)	1m 6s
Test (./vesta)	1m 6s
Test (./bw6_767)	3m 54s
Test (./bls12_381)	3m 13s
	1h 26m 14s

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