[WIP] Implement EIP-7495: StableContainer

[macladson]

EIP: https://eips.ethereum.org/EIPS/eip-7495

See ssz_derive tests for example of ergonomics.

Outstanding Work

• SSZ
  • Encode
    • StableContainer
    • Profile
  • Decode
    • StableContainer
    • Profile
• TreeHash changes
  • StableContainer
  • Profile

Considerations

• Does not touch the custom encoding/decoding implementation (for example when using #[ssz(with = "some_function")]) so these might not work as expected.
• Need a more ergonomic way to restrict fields to Option.

[delbonis]

What is the status of this PR?

[michaelsproul]

@delbonis We got hard blocked on some issues in our TreeHash/Milhouse library.

The full list of things we tried is:

• Change trait to take self by value tree_hash#23
• Add TreeHashMut tree_hash#24
• Interior mutability milhouse#50

Sorry that those notes are fairly messy, the root of the problem is that our current BeaconState struct doesn't derive TreeHash due to requiring mutable access to its Milhouse list/vector types. We were hoping to fix this before merging the feature, otherwise it's useless for us.