implement SOS serialization in minimal_ssz.py

[protolambda]

Note: PR into master, to get it into v0.6.1. Will make PRs into special dev branch for future v0.6.x updates after v0.6.1

Implemented SOS serialization for minimal_ssz.py (util of the executable spec).

Some notes:

• Container encoding differs slightly from previous implementation semantics: I call the helper function encode_series, since it does lists, vectors and containers. Not just containers.
• Removed unnecessary summing & list concatenation. Worth a few lines to get fast CI + tests.
• The encoding seems funny, but is correct from what I understand. Example outlined below. Edit: funny encoding was just incorrect.
• The spec can be improved somewhat:
  • serialize(sum(fixed_lengths + variable_lengths[:i])) is not just inefficient, it is also missing explicit expression of the expected length of the serialization output. Should be BYTES_PER_LENGTH_OFFSET, which is 4, i.e. uint32, something different from most of the other parts of the spec.
  • typo: Reccursively

[protolambda]

@JustinDrake You worked on the SOS wording in the SSZ spec, and are working with bitfields in PR #1019, what do you think about the encoding example above? Fixed related bug, not necessary anymore, see below.

[protolambda]

Now that I think about it, the 0x0000... actually has to be 0x04000... To account for the offset itself. Can fix that later. Base point still stands: do we actually want it to be there?
Edit: what seemed funny, was really just incorrect. Blame jetlag. At least I catched it, and it's fixed now, so good to merge after review :)

[djrtwo]

Nice!

serialize(sum(fixed_lengths + variable_lengths[:i])) is not just inefficient, it is also missing explicit expression of the expected length of the serialization output. Should be BYTES_PER_LENGTH_OFFSET, which is 4, i.e. uint32, something different from most of the other parts of the spec.

And it appears that the definition of serialize in SSZ takes no explicit type... How do we want to proceed here? Probably just mirror our minimal_ssz with an optional type param.