Support for Fields Inheritance of Serializable Class

[Bhargavasomu]

What was wrong?

We should probably try to make the Fields of Serializable Class be inherited. Or in other words, the new fields which appear in the inherited class should be probably appended to the Fields of the Parent Class.

Issue was referred from RLP ethereum/pyrlp#44

How can it be fixed?

Fill this section in if you know how this could or should be fixed.

[Bhargavasomu]

@hwwhww @jannikluhn may I please know the priority or necessity of this issue in SSZ. Thankyou.

[jannikluhn]

I'd say it's low priority as we won't need proper inheritance until at least Phase 1.