[do not merge] Optimized mul_u64 for v0.5.0 #11

Job	Run time
Check no_std	34s
List directories for parallelizing tests	4s
Check Documentation	47s
Check (stable)	13s
Check Style	4s
Test (stable)	27s
Check (nightly)	30s
Test assembly	2m 15s
Test (nightly)	30s
Test (cp6_782/)	5s
Test (mnt4_753/)	1m 15s
Test (mnt6_753/)	52s
Test (./secp384r1)	44s
Test (./ed_on_mnt4_298)	32s
Test (./mnt6_298)	37s
Test (./mnt4_298)	39s
Test (./grumpkin)	45s
Test (./ed_on_bls12_381)	45s
Test (./ed_on_bls12_377)	38s
Test (./secp256r1)	33s
Test (./bw6_761)	37s
Test (./pallas)	43s
Test (./ed25519)	45s
Test (./bls12_381)	49s
Test (./ed_on_bw6_761)	30s
Test (./curve25519)	43s
Test (./bls12_377)	44s
Test (./secp256k1)	42s
Test (./ed_on_bn254)	2m 13s
Test (./secq256k1)	40s
Test (./ed_on_cp6_782)	35s
Test (./vesta)	38s
Test (./ed_on_bls12_381_bandersnatch)	35s
Test (./bn254)	46s
Test (./bw6_767)	32s
Test (./ed_on_mnt4_753)	36s
	24m 57s

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