Union type update

[protolambda]

• Clarify missing Union type in SSZ spec
• Use "selector" wording instead of "type", since there may be multiple options of the same type in rare cases.
• Reduce selector to 1 byte, and reserve the top 128 values (most significant bit) for future backwards-compatible extensions
• Fix null reference in python-ish spec. It's referenced as the native-none/nil/null of the implementation anyway.
• Update serialization and merkleization to cover None case in the Union

Note: this change is assumed to be backwards-compatible, since Union is not used anywhere in the spec. Outside of the spec there is one known case in the DB format of Lighthouse, but the old serialization/deserialization can be kept around there.

TODO:

• See Implement SSZ Union type, with parametrized selector byte length protolambda/remerkleable#9 for remerkleable update. If I get an ACK on this PR, I will make a new remerkleable release, and update the pyspec, so we can start using Union in the spec and testing.
  • Update: see remerkleable v0.1.20

fixes #2270

[protolambda]

Tagging this with the Altair label. It's not critical, nor used in consensus, but I like to ship the SSZ update before the old Union type gets more actual uses outside of the spec. And we are looking to use it in the Merge and Sharding specs.

[Nashatyrev]

LTGM

[ralexstokes]

looks great! now we can define Option types in the spec and i'm sure there will be other uses along the way

left one comment below for clarification