Fix the missing `parallel` feature flag for ark-serialize #2867

Job	Run time
Check (stable)	16s
List directories for parallelizing tests	5s
Check (nightly)	1m 11s
Check Documentation	1m 3s
Check Style	5s
Test (stable)	9m 19s
Test (cp6_782/)	10s
Test (nightly)	17m 45s
Test (mnt4_753/)	11s
Test (mnt6_753/)	11s
Check no_std	28s
Test assembly	8m 34s
Test (./bw6_761)	4m 11s
Test (./ed_on_mnt4_298)	1m 8s
Test (./mnt6_298)	5m 32s
Test (./mnt4_298)	3m 46s
Test (./secp256r1)	1m 1s
Test (./pallas)	1m 5s
Test (./secp384r1)	1m 26s
Test (./vesta)	1m 25s
Test (./ed_on_bn254)	1m 6s
Test (./ed25519)	54s
Test (./ed_on_mnt4_753)	3m 56s
Test (./ed_on_bls12_381)	1m 13s
Test (./ed_on_bw6_761)	59s
Test (./bw6_767)	4m 10s
Test (./ed_on_cp6_782)	1m 18s
Test (./curve25519)	1m 10s
Test (./grumpkin)	1m 12s
Test (./secp256k1)	1m 6s
Test (./secq256k1)	1m 4s
Test (./ed_on_bls12_377)	1m 22s
Test (./ed_on_bls12_381_bandersnatch)	1m 2s
Test (./bn254)	2m 21s
Test (./bls12_377)	4m 14s
Test (./bls12_381)	3m 28s
	1h 29m 27s

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