ff: simplify `from_bigint` conversion for Fp #3035

Job	Run time
Check Documentation	52s
Check (stable)	7s
Check Style	8s
Test (stable)	8m 53s
Check (nightly)	1m 14s
Test (nightly)	17m 48s
Check no_std	23s
List directories for parallelizing tests	3s
Test assembly	8m 31s
Test (cp6_782/)	7s
Test (mnt4_753/)	10m 11s
Test (mnt6_753/)	12s
Test (./ed_on_mnt4_753)	3m 51s
Test (./secp256r1)	57s
Test (./ed_on_mnt4_298)	1m 6s
Test (./ed_on_cp6_782)	1m 20s
Test (./mnt4_298)	3m 41s
Test (./mnt6_298)	5m 24s
Test (./ed_on_bn254)	59s
Test (./bn254)	2m 14s
Test (./secp256k1)	1m 4s
Test (./bw6_761)	4m 4s
Test (./bls12_377)	4m 5s
Test (./grumpkin)	1m 5s
Test (./secp384r1)	1m 23s
Test (./ed_on_bls12_377)	1m 7s
Test (./secq256k1)	56s
Test (./ed25519)	57s
Test (./ed_on_bls12_381)	1m 12s
Test (./ed_on_bw6_761)	37s
Test (./curve25519)	1m 1s
Test (./ed_on_bls12_381_bandersnatch)	1m 1s
Test (./pallas)	1m 12s
Test (./vesta)	1m 8s
Test (./bw6_767)	4m 8s
Test (./bls12_381)	3m 13s
	1h 36m 14s

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