ec: small refactor for scalar_decomposition (#898) #3103

Job	Run time
Test (stable)	9m 0s
Test (nightly)	17m 32s
List directories for parallelizing tests	2s
Check Documentation	1m 1s
Check (stable)	14s
Check Style	12s
Check (nightly)	1m 13s
Test assembly	8m 24s
Test (cp6_782/)	6s
Test (mnt4_753/)	9m 33s
Test (mnt6_753/)	11s
Check no_std	31s
Test (./ed_on_mnt4_753)	3m 58s
Test (./secp256r1)	54s
Test (./ed_on_mnt4_298)	1m 4s
Test (./ed_on_cp6_782)	1m 12s
Test (./mnt4_298)	3m 59s
Test (./mnt6_298)	5m 20s
Test (./ed_on_bn254)	56s
Test (./bn254)	2m 14s
Test (./secp256k1)	1m 0s
Test (./bw6_761)	3m 48s
Test (./bls12_377)	4m 14s
Test (./grumpkin)	1m 15s
Test (./secp384r1)	1m 16s
Test (./ed_on_bls12_377)	1m 2s
Test (./secq256k1)	54s
Test (./ed25519)	55s
Test (./ed_on_bls12_381)	1m 15s
Test (./ed_on_bw6_761)	49s
Test (./curve25519)	1m 1s
Test (./ed_on_bls12_381_bandersnatch)	59s
Test (./pallas)	1m 15s
Test (./vesta)	1m 16s
Test (./bw6_767)	3m 59s
Test (./bls12_381)	3m 16s
	1h 35m 50s

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