Fix the missing `parallel` feature flag for ark-serialize, take #2 #2868

Job	Run time
List directories for parallelizing tests	3s
Check no_std	30s
Check (stable)	9s
Check (nightly)	1m 10s
Check Style	16s
Check Documentation	1m 0s
Test (stable)	9m 11s
Test assembly	8m 33s
Test (nightly)	17m 49s
Test (cp6_782/)	28m 46s
Test (mnt4_753/)	9m 48s
Test (mnt6_753/)	21m 38s
Test (./ed25519)	1m 1s
Test (./mnt6_298)	5m 46s
Test (./grumpkin)	1m 6s
Test (./ed_on_cp6_782)	1m 26s
Test (./ed_on_mnt4_753)	3m 44s
Test (./ed_on_bw6_761)	43s
Test (./bls12_381)	3m 13s
Test (./ed_on_bls12_381_bandersnatch)	1m 5s
Test (./ed_on_mnt4_298)	1m 7s
Test (./secq256k1)	1m 1s
Test (./bn254)	2m 17s
Test (./mnt4_298)	3m 51s
Test (./secp256r1)	59s
Test (./vesta)	1m 7s
Test (./ed_on_bn254)	1m 0s
Test (./bw6_761)	3m 59s
Test (./curve25519)	1m 21s
Test (./pallas)	1m 6s
Test (./ed_on_bls12_377)	1m 9s
Test (./secp384r1)	1m 19s
Test (./secp256k1)	1m 3s
Test (./ed_on_bls12_381)	1m 23s
Test (./bls12_377)	4m 12s
Test (./bw6_767)	4m 0s
	2h 27m 51s

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