Fuzzing utilities package / SSZ decoding for spec

[protolambda]

SSZ decoding through pyssz for fuzzing. Intended to use with fuzzing. But maybe it's better to make a generic toolbelt package?

Implemented:

• translate a pyspec type to a py-ssz sedes (serialization description?)
• translate a py-ssz decoded value to a pyspec value

Usage:

from eth2spec.phase0 import spec
from preset_loader import loader
from eth2spec.utils.ssz import ssz_impl as spec_ssz_impl

configs_path = "../../configs/"
config_name = "minimal"
presets = loader.load_presets(configs_path, config_name)
spec.apply_constants_preset(presets)

typ = spec.BeaconBlock
v = typ()
v.slot = 123

data = spec_ssz_impl.serialize(v)
print('data', data.hex())

block_sedes = translate_typ(typ)
print('sedes', block_sedes)

raw_value = block_sedes.deserialize(data)
print('raw_value', raw_value)

assert block_sedes.serialize(raw_value) == data

block = translate_value(raw_value, typ)
print('res', block)

assert spec_ssz_impl.hash_tree_root(v) == spec_ssz_impl.hash_tree_root(block)

TODO:

• decide on packaging (fuzzing? toolbelt?) Moved into pyspec, since it closely works with py-ssz, and needs to be readily accessible for Guido to start fuzzing
• add setup.py so it can be imported into other modules (see above)
• update py-ssz dependency version, this version does not use offsets, but it looks like the newest unreleased code on github does

[protolambda]

@hwwhww or @carver can you help me out with a new py-ssz release to read offsets in decoding? (I see offsets in the py-ssz github code, is that working?) 🙏

[carver]

I'm not deeply familiar with py-ssz, I seem to be a pypi maintainer only as emergency backup. Maybe @pipermerriam could help here with the question about read offsets (and a release)?

[protolambda]

Blocked by ethereum/py-ssz#74

[protolambda]

This PR is ready for review