[WIP] Implement EIP-7495: StableContainer

[macladson]

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

Related PR: sigp/ethereum_ssz#16

[michaelsproul]

Just pushed a smol commit (5c10761) to fix the duplicate dependency issues. The problem was the non-tree_hash crates were pulling in the git version of tree_hash, which cargo treats as completely separate from the current crate (i.e. the version of tree_hash currently checked out). The fix is to use patch to replace the git version of the crate with the local one.

Long-term I don't think this will be needed, as there'll be proper releases to depend on. Although I do wonder if there's a circular dependency now? ssz_types depends on tree_hash, but tree_hash_derive also dev-depends on ssz_types, and tree_hash dev-depends on tree_hash_derive... Maybe the fact that some of the deps are dev-dependencies makes it OK, but it kind of makes it harder to release tree_hash + tree_hash_derive together, as I think the dep chain (ignoring dev-dependencies) is now:

tree_hash <- ssz_types <- tree_hash_derive

and our release strategy previously was:

• publish tree_hash_derive
• publish tree_hash
• publish ssz_types

whereas now it might have to be:

• publish tree_hash
• publish ssz_types
• publish tree_hash_derive

[ncitron]

This assertion seems to prevent you from using superstruct with a transparent (or transparent_stable) tree_hash marker where one variant is a normal container and the other is a profile. This seems like a useful thing to do. Is there a way around this?