ff: some simplifications for biginteger #3038

Job	Run time
Check Style	8s
Check no_std	23s
Check (stable)	9s
List directories for parallelizing tests	3s
Check (nightly)	1m 15s
Check Documentation	53s
Test (stable)	8m 58s
Test assembly	8m 30s
Test (nightly)	17m 33s
Test (cp6_782/)	10s
Test (mnt4_753/)	15m 49s
Test (mnt6_753/)	13s
Test (./ed_on_mnt4_753)	3m 51s
Test (./secp256r1)	57s
Test (./ed_on_mnt4_298)	1m 6s
Test (./ed_on_cp6_782)	1m 14s
Test (./mnt4_298)	3m 44s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	58s
Test (./bn254)	2m 14s
Test (./secp256k1)	1m 2s
Test (./bw6_761)	3m 58s
Test (./bls12_377)	4m 6s
Test (./grumpkin)	1m 8s
Test (./secp384r1)	1m 17s
Test (./ed_on_bls12_377)	1m 5s
Test (./secq256k1)	53s
Test (./ed25519)	1m 1s
Test (./ed_on_bls12_381)	1m 15s
Test (./ed_on_bw6_761)	37s
Test (./curve25519)	1m 4s
Test (./ed_on_bls12_381_bandersnatch)	1m 3s
Test (./pallas)	1m 4s
Test (./vesta)	1m 8s
Test (./bw6_767)	3m 59s
Test (./bls12_381)	3m 11s
	1h 41m 25s

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