Change genesis.code encoding in chainspec to bytes.

[tomusdrw]

Currently genesis.code inside chain spec JSON is encoded as raw Vec<u8> which results in a JSON array containing shitload of numbers, which by default when pretty-printed results in one number per line.

Instead we should encode code as hex (see Bytes wrapper) as it's way more compact, but to maintain backward compatibility we should support both formats when deserializing the chain spec.

CC @JoshOrndorff, I saw that on your seminar and thought that we could fix it :)

[bkchr]

Wasn't this fixed by this: #4025 ?

[tomusdrw]

@bkchr Good point, it's quite possible as that's what the title suggests. Let me confirm that and close that issue if that's the case.

[tomusdrw]

Confirming it's already fixed in #4025.