serialize: add unit tests for serialize impls #3049

Job	Run time
Check (stable)	1s
Check (nightly)	1s
Check no_std	32s
List directories for parallelizing tests	3s
Check Documentation	54s
Check Style	7s
Test (stable)	8m 56s
Test (nightly)	17m 30s
Test assembly	8m 31s
Test (cp6_782/)	9s
Test (mnt4_753/)	13s
Test (mnt6_753/)	12s
Test (./ed_on_mnt4_753)	3m 51s
Test (./secp256r1)	56s
Test (./ed_on_mnt4_298)	1m 3s
Test (./ed_on_cp6_782)	1m 15s
Test (./mnt4_298)	3m 56s
Test (./mnt6_298)	5m 23s
Test (./ed_on_bn254)	1m 0s
Test (./bn254)	2m 22s
Test (./secp256k1)	1m 5s
Test (./bw6_761)	3m 58s
Test (./bls12_377)	4m 9s
Test (./grumpkin)	1m 10s
Test (./secp384r1)	1m 19s
Test (./ed_on_bls12_377)	1m 10s
Test (./secq256k1)	52s
Test (./ed25519)	58s
Test (./ed_on_bls12_381)	1m 20s
Test (./ed_on_bw6_761)	38s
Test (./curve25519)	1m 12s
Test (./ed_on_bls12_381_bandersnatch)	1m 8s
Test (./pallas)	1m 4s
Test (./vesta)	1m 3s
Test (./bw6_767)	4m 1s
Test (./bls12_381)	3m 21s
	1h 25m 23s

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