[do not merge] Optimized mul_u64 for v0.5.0 #12

Job	Run time
Check (stable)	8s
List directories for parallelizing tests	5s
Check (nightly)	29s
Check Documentation	46s
Check no_std	22s
Check Style	8s
Test (stable)	38s
Test (nightly)	33s
Test assembly	1m 10s
Test (cp6_782/)	10s
Test (mnt4_753/)	46s
Test (mnt6_753/)	1m 1s
Test (./secp384r1)	32s
Test (./ed_on_mnt4_298)	36s
Test (./mnt6_298)	40s
Test (./mnt4_298)	46s
Test (./grumpkin)	42s
Test (./ed_on_bls12_381)	45s
Test (./ed_on_bls12_377)	41s
Test (./secp256r1)	35s
Test (./bw6_761)	38s
Test (./pallas)	39s
Test (./ed25519)	38s
Test (./bls12_381)	33s
Test (./ed_on_bw6_761)	36s
Test (./curve25519)	40s
Test (./bls12_377)	48s
Test (./secp256k1)	43s
Test (./ed_on_bn254)	40s
Test (./secq256k1)	34s
Test (./ed_on_cp6_782)	38s
Test (./vesta)	44s
Test (./ed_on_bls12_381_bandersnatch)	36s
Test (./bn254)	48s
Test (./bw6_767)	32s
Test (./ed_on_mnt4_753)	33s
	21m 53s

Provide feedback