Relax `Arc` cfg; remove `Send` bound from `Valid`; implement `Valid` & `CanonicalDeserialize` for `Rc` #2835

Job	Run time
Check (stable)	18s
Check no_std	30s
Check (nightly)	1m 20s
Check Documentation	1m 0s
List directories for parallelizing tests	3s
Check Style	12s
Test (stable)	10m 14s
Test assembly	8m 42s
Test (cp6_782/)	6s
Test (nightly)	18m 20s
Test (mnt4_753/)	19m 22s
Test (mnt6_753/)	12s
Test (./ed_on_bls12_377)	1m 10s
Test (./ed_on_bls12_381)	1m 10s
Test (./ed_on_bw6_761)	44s
Test (./vesta)	1m 15s
Test (./ed25519)	55s
Test (./ed_on_bn254)	59s
Test (./ed_on_cp6_782)	1m 21s
Test (./mnt4_298)	3m 48s
Test (./ed_on_mnt4_753)	3m 47s
Test (./secp384r1)	1m 25s
Test (./curve25519)	1m 3s
Test (./secp256k1)	1m 15s
Test (./secq256k1)	54s
Test (./ed_on_bls12_381_bandersnatch)	1m 11s
Test (./bn254)	2m 21s
Test (./secp256r1)	1m 3s
Test (./bls12_381)	3m 24s
Test (./pallas)	1m 15s
Test (./bw6_761)	4m 0s
Test (./grumpkin)	1m 23s
Test (./ed_on_mnt4_298)	1m 5s
Test (./mnt6_298)	5m 38s
Test (./bw6_767)	3m 51s
Test (./bls12_377)	4m 13s
	1h 49m 29s

Provide feedback