SSZ static testing [blocked by #960]

[protolambda]

See /specs/test_formats/ssz_static/README.md and /specs/test_formats/ssz_generic/README.md to learn about the two types of SSZ testing.

SSZ itself is very much the same. It's just that the clients build towards efficient SSZ deserialization/serialization for their phase-0 structs, and generic SSZ would not cover this code.

Also, the generic SSZ test-suite was build on by:

• [WIP] SSZ update eth2.0-test-generators#32
• Tree Hash Test Generator eth2.0-test-generators#34

But these are based on the older SSZ (afaik). Production-first clients have a high need to just test spec-SSZ for their ETH-2-phase0 types.

Generic SSZ would require to:

• standardize a SSZ type format
• write a type parser in every client, and an encoder for the test-gen
• make clients instantiate these dynamic types. Something you wouldn't do in Go or Rust. Instead, a generic Container/Vector/List object would be used, but this makes things less efficient, and introduces more overhead in the SSZ encoder/decoder.
  I do see generic-SSZ be a thing in the future, but for now we really just need testing for the ETH-2.0 types, that every client is looking to serialize/deserialize anyway.

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The introduced tests:

For every type pulled from the phase-0 spec (dynamic, generator does not need to be updated), create a bunch of random instances (we use the SSZ typing to fill it with random data), and output test vectors for it.
The randomization has different modes, and a "chaos" switch to randomly change the mode during filling of the SSZ instance with random data.
(Note: some modes are more like edge-cases, e.g. all zeroes, and will only output 1 case, unless in chaos-mode and things are mixed up)

The test-format itself is documented in the ssz_static test-format docs included in the PR.

It produces roughly ~10K test-cases, split in different suites. Most are for the minimal testing-configuration. And a few random ones are generated for the mainnet-configuration (larger objects, requires more space).

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Note: this PR is branched of the ssz-impl-fixes (PR #960), to make use of the new fixed spec-conformant SSZ implementation. Wait for that PR to merge.

[djrtwo]

Looks good. Agreed with static/generic split for now
Just a few minor comments

Curious -- how large is the ~10k test case output?

[protolambda]

@djrtwo Still relatively small compared to the Eth 1.0 tests repo. Just 67MB, fine with git LFS.

du -sh yaml_tests/ssz_static
67M     yaml_tests/ssz_static

[protolambda]

Rebased on dev to solve merge conflicts (this PR was based of my previous SSZ-PR, which still had some changes / rebases before being submitted). Also applied suggestions from Danny 👍

[djrtwo]

lgtm