Refactor and write tests for Simple Serialization (SSZ)

[hwwhww]

What's wrong?

Currently, the tests are at tests/utils/test_serialization.py, and it's using ActiveState and PartialCrosslinkRecord classes for testing. It will be nice to use some testing-only Foo classes to cover more testing cases instead of having a dependency of beacon_chain.

How can it be fixed

Write new tests at tests/ssz/test_*.py. Cover all test cases. Includes:

1. Add tests of serialization and deserialization of every type (ref: https://github.com/sigp/lighthouse/tree/ssz-readme/ssz#serializeencode)
2. Refactor SSZ module
   • Error handling
   • Does it make sense to make a Serializable class like RLP?

[p-mc2]

Basic tests moved to tests/ssz.

Beginning to (1.) write tests for missing types, will update when moved on to (2.) refactoring SSZ to avoid duplicate work in case someone else decides to work on that part while I'm working on (1.). Cheers.

[p-mc2]

All of my tests have been failing for lists. Has anyone successfully tested the serialization for lists? Maybe I am wrong by assuming [3, 4, 5] --> [b'\x03', b'\x04', b'\x05']

[hwwhww]

Oh, sorry for that I missed it. The minimum int length is 8, so we have:

serialize([3, 4, 5], ['int8'])
b'\x00\x00\x00\x03\x03\x04\x05'

[p-mc2]

Ah! That makes a lot more sense. Thanks for the assist!