Improve widening mul operations' performance on wasm (#828) #2894

Job	Run time
Check Style	10s
Check (stable)	12s
Check Documentation	1m 0s
Check (nightly)	1m 9s
Test (stable)	9m 16s
Test (nightly)	18m 3s
Check no_std	49s
List directories for parallelizing tests	3s
Test assembly	8m 44s
Test (cp6_782/)	5s
Test (mnt4_753/)	18s
Test (mnt6_753/)	13s
Test (./ed25519)	1m 15s
Test (./mnt6_298)	5m 42s
Test (./grumpkin)	1m 24s
Test (./ed_on_cp6_782)	1m 15s
Test (./ed_on_mnt4_753)	3m 49s
Test (./ed_on_bw6_761)	50s
Test (./bls12_381)	3m 15s
Test (./ed_on_bls12_381_bandersnatch)	1m 7s
Test (./ed_on_mnt4_298)	1m 18s
Test (./secq256k1)	58s
Test (./bn254)	2m 42s
Test (./mnt4_298)	3m 46s
Test (./secp256r1)	1m 8s
Test (./vesta)	1m 19s
Test (./ed_on_bn254)	1m 3s
Test (./bw6_761)	3m 50s
Test (./curve25519)	1m 7s
Test (./pallas)	1m 16s
Test (./ed_on_bls12_377)	1m 19s
Test (./secp384r1)	1m 31s
Test (./secp256k1)	1m 13s
Test (./ed_on_bls12_381)	1m 36s
Test (./bls12_377)	4m 15s
Test (./bw6_767)	3m 59s
	1h 30m 59s

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