chore: add back ssz feature #523

Job	Run time
feature-checks	1m 17s
docs	45s
clippy	40s
fmt	11s
wasm	33s
test stable --no-default-features	1m 34s
test stable	1m 51s
test stable --all-features	1m 38s
test beta --no-default-features	1m 11s
test beta	1m 5s
test beta --all-features	1m 23s
test nightly --no-default-features	1m 6s
test nightly	1m 6s
test nightly --all-features	1m 22s
test 1.68 --no-default-features	44s
test 1.68	47s
	17m 13s

Provide feedback