Support SSZ StableContainer

[dapplion]

Implement

• EIP-7495: SSZ StableContainer
• EIP-7688: Forward compatible consensus data structures

Mostly update underlying lib https://github.com/sigp/tree_hash to support StableContainer merkleization and have some nice DX way to specify property ordering

[michaelsproul]

In progress here:

• [WIP] Implement EIP-7495: StableContainer ethereum_ssz#16
• [WIP] Implement EIP-7495: StableContainer tree_hash#15