Beacon chain types refactor (#3995)

massive refactor to our beacon chain types. we are now aware of all
forks and forced to handle them.

also versioned the ethereum client state, potentially will do the same
to the consensus state?

includes the changes from #3878 as well.

TODO:

- [ ] docs on new libraries
- [ ] json spec tests for the beacon types (not a hard requirement for
merging this pr)