Implementation of `core::ops::Shl` and `core::ops::ShlAssign` to deprecate `BigInteger::muln()` #2577

Job	Run time
Check Documentation	55s
Check Style	7s
Test (stable)	5m 18s
Test (nightly)	10m 53s
Check (stable)	16s
Check (nightly)	1m 11s
Check no_std	39s
List directories for parallelizing tests	4s
Test assembly	5m 22s
Test (cp6_782/)	11s
Test (mnt4_753/)	17s
Test (mnt6_753/)	14s
Test (./bw6_761)	2m 50s
Test (./ed_on_mnt4_298)	1m 0s
Test (./ed_on_mnt4_753)	1m 38s
Test (./pallas)	57s
Test (./bn254)	1m 57s
Test (./ed_on_cp6_782)	1m 2s
Test (./ed25519)	1m 5s
Test (./secq256k1)	44s
Test (./ed_on_bls12_381)	1m 8s
Test (./secp256k1)	1m 4s
Test (./grumpkin)	59s
Test (./bls12_377)	3m 27s
Test (./ed_on_bw6_761)	41s
Test (./curve25519)	1m 8s
Test (./mnt4_298)	2m 3s
Test (./bls12_381)	2m 49s
Test (./secp384r1)	1m 0s
Test (./mnt6_298)	2m 59s
Test (./bw6_767)	2m 55s
Test (./secp256r1)	47s
Test (./ed_on_bls12_377)	1m 16s
Test (./ed_on_bls12_381_bandersnatch)	57s
Test (./ed_on_bn254)	55s
Test (./vesta)	1m 2s
	1h 1m 50s

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