Relax `Arc` cfg; remove `Send` bound from `Valid`; implement `Valid` & `CanonicalDeserialize` for `Rc` #2839

Job	Run time
Check (stable)	15s
Test (stable)	9m 19s
Test (nightly)	17m 59s
Check (nightly)	1m 18s
List directories for parallelizing tests	2s
Check Documentation	50s
Test (cp6_782/)	7s
Check Style	9s
Test (mnt4_753/)	12s
Check no_std	24s
Test (mnt6_753/)	33m 24s
Test assembly	8m 32s
Test (./secq256k1)	54s
Test (./ed_on_mnt4_753)	3m 49s
Test (./vesta)	1m 5s
Test (./ed_on_cp6_782)	1m 13s
Test (./secp384r1)	1m 14s
Test (./bls12_377)	4m 15s
Test (./ed_on_bls12_381)	1m 18s
Test (./pallas)	1m 3s
Test (./ed_on_bw6_761)	35s
Test (./ed_on_bls12_381_bandersnatch)	1m 2s
Test (./mnt4_298)	3m 45s
Test (./secp256k1)	1m 2s
Test (./bn254)	2m 27s
Test (./bls12_381)	3m 14s
Test (./ed_on_bls12_377)	1m 8s
Test (./bw6_761)	4m 0s
Test (./ed25519)	52s
Test (./ed_on_bn254)	57s
Test (./mnt6_298)	5m 28s
Test (./grumpkin)	1m 14s
Test (./bw6_767)	3m 59s
Test (./ed_on_mnt4_298)	1m 16s
Test (./curve25519)	1m 6s
Test (./secp256r1)	1m 4s
	2h 0m 31s

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