Implementation of `core::ops::Shl` and `core::ops::ShlAssign` to deprecate `BigInteger::muln()` #2576

Job	Run time
Check no_std	25s
Check (stable)	11s
Check (nightly)	1m 13s
List directories for parallelizing tests	5s
Check Documentation	52s
Check Style	8s
Test (stable)	5m 20s
Test assembly	5m 15s
Test (nightly)	10m 50s
Test (cp6_782/)	6s
Test (mnt4_753/)	14s
Test (mnt6_753/)	22s
Test (./bw6_761)	2m 39s
Test (./ed_on_mnt4_298)	1m 1s
Test (./ed_on_mnt4_753)	1m 44s
Test (./pallas)	1m 2s
Test (./bn254)	1m 48s
Test (./ed_on_cp6_782)	57s
Test (./ed25519)	59s
Test (./secq256k1)	47s
Test (./ed_on_bls12_381)	1m 5s
Test (./secp256k1)	54s
Test (./grumpkin)	1m 0s
Test (./bls12_377)	3m 28s
Test (./ed_on_bw6_761)	36s
Test (./curve25519)	57s
Test (./mnt4_298)	2m 7s
Test (./bls12_381)	2m 56s
Test (./secp384r1)	50s
Test (./mnt6_298)	2m 58s
Test (./bw6_767)	2m 59s
Test (./secp256r1)	52s
Test (./ed_on_bls12_377)	1m 3s
Test (./ed_on_bls12_381_bandersnatch)	53s
Test (./ed_on_bn254)	48s
Test (./vesta)	58s
	1h 0m 22s

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