[WIP] SSZ update

[jannikluhn]

This PR replaces the existing oudated SSZ test generators. It includes tests for all types defined in the 0.5 spec release, excluding aliases. There are no tests for invalid inputs or outputs.

It will likely take a while until we'd be able to merge this PR as

• it depends on a bunch of unmerged PRs in py-ssz
• we'll migrate the generators to the spec repo (see Proposal for restructure to support CI tests, pyspec, and better test generation flow. consensus-specs#836)

Therefore, I opened it mostly for reference and to give people a chance to review and/or complain if they find an error in the tests.

See this gist for the generated tests vectors.

[jannikluhn]

(At least) 15 of the 730 test are broken. I think all of them are related to this issue: ethereum/consensus-specs#854

Here's a list with filenames and line numbers:

deeply_nested_composite_types.yaml:387
deeply_nested_composite_types.yaml:428
deeply_nested_composite_types.yaml:867
deeply_nested_composite_types.yaml:1164
nested_composite_types.yaml:4256
nested_composite_types.yaml:4276
nested_composite_types.yaml:4323
nested_composite_types.yaml:5460
nested_composite_types.yaml:5965
nested_composite_types.yaml:6191
nested_composite_types.yaml:9191
nested_composite_types.yaml:9320
nested_composite_types.yaml:9463
nested_composite_types.yaml:9473
nested_composite_types.yaml:10121

[hwwhww]

It's a little awkward that "Vector" is "Tuple" in spec v0.5.1 (https://github.com/ethereum/eth2.0-specs/blob/v0.5.1/specs/simple-serialize.md), and will be renamed in the next release. IMO it's okay but we should note that in the py-ssz and tests release.

[protolambda]

Concern here is that this uses a 2 months old SSZ package, and incompatible with the SSZ typing definitions used in the spec. See https://github.com/ethereum/eth2.0-specs/blob/d7d2eeadb3e8dad3fed51b243e9647c64c084136/specs/test_formats/ssz_generic/README.md and ethereum/consensus-specs#962 for context.