ff: simplify `from_base_prime_field_elems` #3037

Job	Run time
Check (stable)	12s
Check (nightly)	1m 8s
Check Documentation	53s
List directories for parallelizing tests	4s
Check Style	7s
Test (stable)	8m 56s
Test (nightly)	17m 35s
Check no_std	27s
Test assembly	8m 20s
Test (cp6_782/)	5s
Test (mnt4_753/)	12m 20s
Test (mnt6_753/)	15s
Test (./ed_on_mnt4_753)	3m 50s
Test (./secp256r1)	56s
Test (./ed_on_mnt4_298)	1m 5s
Test (./ed_on_cp6_782)	1m 12s
Test (./mnt4_298)	3m 42s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	59s
Test (./bn254)	2m 13s
Test (./secp256k1)	1m 3s
Test (./bw6_761)	3m 46s
Test (./bls12_377)	4m 12s
Test (./grumpkin)	1m 14s
Test (./secp384r1)	1m 22s
Test (./ed_on_bls12_377)	1m 6s
Test (./secq256k1)	54s
Test (./ed25519)	57s
Test (./ed_on_bls12_381)	1m 8s
Test (./ed_on_bw6_761)	41s
Test (./curve25519)	1m 2s
Test (./ed_on_bls12_381_bandersnatch)	1m 2s
Test (./pallas)	1m 8s
Test (./vesta)	1m 24s
Test (./bw6_767)	3m 59s
Test (./bls12_381)	3m 11s
	1h 37m 54s

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