Correctly serialize code in chain spec as hex (#4025)

* Correctly serialize code in chain spec as hex.

Due to a bug, the runtime code was previously serialized as a JSON
array of numbers, pretty printed one byte per line.

* Remove panic in macro and whitelist attribute types for storage
genesis config lines.

* Use syn::Error to enforce whitelisted attributes on genesis config.

* Blacklist genesis extra config line attributes instead of whitelist.