Work-in-progress on SSZ partials

[vbuterin]

Now updated to the latest spec.

[protolambda]

#1180 Is pretty much ready, tests pass. (fully structured/typed SSZ, less lines 🎉) Just final review + verify SSZ behavior through generator output comparison.
I rebased the partials out of 1180, since this PR is here for those now. The updates to the partials made in 1180 (use of typing, fixes for linting) can be found on the new-partials-updated branch. See commit 58ffa41

After merging the above commit into here, and when 1180 is merged into dev, this PR can complete the adoption of the new SSZ (rebase on dev, drop changes to ssz implementation, just keep partials), and then we can merge this too :)

[protolambda]

Going to rebase this on new dev branch, then add some things to improve the experience with generalized indices with path-like syntax sugar, as I dislike the array of keys thing.

[protolambda]

This PR is largely outdated, an during devcon discussions about SSZ standardization and light client work there is a motivation to pick this up again, and move it to a separate repository for various reasons. See final comment on #1261, closing this PR now.