[do not merge] Optimized mul_u64 for v0.5.0 #15

Job	Run time
Check no_std	27s
List directories for parallelizing tests	4s
Check Documentation	50s
Check (stable)	8s
Check (nightly)	30s
Check Style	7s
Test (stable)	32s
Test assembly	1m 14s
Test (nightly)	30s
Test (cp6_782/)	11s
Test (mnt4_753/)	45s
Test (mnt6_753/)	1m 0s
Test (./secp384r1)	32s
Test (./ed_on_mnt4_298)	46s
Test (./mnt6_298)	46s
Test (./mnt4_298)	45s
Test (./grumpkin)	44s
Test (./ed_on_bls12_381)	39s
Test (./ed_on_bls12_377)	40s
Test (./secp256r1)	34s
Test (./bw6_761)	48s
Test (./pallas)	41s
Test (./ed25519)	34s
Test (./bls12_381)	44s
Test (./ed_on_bw6_761)	37s
Test (./curve25519)	40s
Test (./bls12_377)	45s
Test (./secp256k1)	39s
Test (./ed_on_bn254)	32s
Test (./secq256k1)	33s
Test (./ed_on_cp6_782)	45s
Test (./vesta)	42s
Test (./ed_on_bls12_381_bandersnatch)	36s
Test (./bn254)	40s
Test (./bw6_767)	45s
Test (./ed_on_mnt4_753)	34s
	22m 19s

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