Add Optional type

[dapplion]

See https://eips.ethereum.org/EIPS/eip-6475

Used by Verge spec feature https://github.com/ethereum/consensus-specs/pull/3230/files#diff-96e72ffddefb3d85f86dd7cb9f7a75defae9a226db75353b2e2b5b4a01ef685dR117

[nflaig]

assign me

[jflo]

Are you still working on this @nflaig ? I would be willing to add to an existing branch if there is one.

[nflaig]

@jflo currently not working on this, feel free to open a PR

[nflaig]

Related spec update ethereum/consensus-specs#3336

[etan-status]

Note: It is still being discussed whether Optional will be more optimized, or whether it will just be a type alias for Union. See https://ethereum-magicians.org/t/eip-6475-ssz-optional/12891

[nflaig]

Thanks @etan-status, will definitely follow that discussion

[nflaig]

Resolved by #329

[draghtnight]

See https://eips.ethereum.org/EIPS/eip-6475

Used by Verge spec feature https://github.com/ethereum/consensus-specs/pull/3230/files#diff-96e72ffddefb3d85f86dd7cb9f7a75defae9a226db75353b2e2b5b4a01ef685dR117