Implement SSZ union type (#2579)

## Issue Addressed

NA

## Proposed Changes

Implements the "union" type from the SSZ spec for `ssz`, `ssz_derive`, `tree_hash` and `tree_hash_derive` so it may be derived for `enums`:

https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/ssz/simple-serialize.md#union

The union type is required for the merge, since the `Transaction` type is defined as a single-variant union `Union[OpaqueTransaction]`.

### Crate Updates

This PR will (hopefully) cause CI to publish new versions for the following crates:

- `eth2_ssz_derive`: `0.2.1` -> `0.3.0`
- `eth2_ssz`: `0.3.0` -> `0.4.0`
- `eth2_ssz_types`: `0.2.0` -> `0.2.1`
- `tree_hash`: `0.3.0` -> `0.4.0`
- `tree_hash_derive`: `0.3.0` -> `0.4.0`

These these crates depend on each other, I've had to add a workspace-level `[patch]` for these crates. A follow-up PR will need to remove this patch, ones the new versions are published.

### Union Behaviors

We already had SSZ `Encode` and `TreeHash` derive for enums, however it just did a "transparent" pass-through of the inner value. Since the "union" decoding from the spec is in conflict with the transparent method, I've required that all `enum` have exactly one of the following enum-level attributes:

#### SSZ

-  `#[ssz(enum_behaviour = "union")]`
    - matches the spec used for the merge
-  `#[ssz(enum_behaviour = "transparent")]`
    - maintains existing functionality
    - not supported for `Decode` (never was)
    
#### TreeHash

-  `#[tree_hash(enum_behaviour = "union")]`
    - matches the spec used for the merge
-  `#[tree_hash(enum_behaviour = "transparent")]`
    - maintains existing functionality

This means that we can maintain the existing transparent behaviour, but all existing users will get a compile-time error until they explicitly opt-in to being transparent.

### Legacy Option Encoding

Before this PR, we already had a union-esque encoding for `Option<T>`. However, this was with the *old* SSZ spec where the union selector was 4 bytes. During merge specification, the spec was changed to use 1 byte for the selector.

Whilst the 4-byte `Option` encoding was never used in the spec, we used it in our database. Writing a migrate script for all occurrences of `Option` in the database would be painful, especially since it's used in the `CommitteeCache`. To avoid the migrate script, I added a serde-esque `#[ssz(with = "module")]` field-level attribute to `ssz_derive` so that we can opt into the 4-byte encoding on a field-by-field basis.

The `ssz::legacy::four_byte_impl!` macro allows a one-liner to define the module required for the `#[ssz(with = "module")]` for some `Option<T> where T: Encode + Decode`.

Notably, **I have removed `Encode` and `Decode` impls for `Option`**. I've done this to force a break on downstream users. Like I mentioned, `Option` isn't used in the spec so I don't think it'll be *that* annoying. I think it's nicer than quietly having two different union implementations or quietly breaking the existing `Option` impl.

### Crate Publish Ordering

I've modified the order in which CI publishes crates to ensure that we don't publish a crate without ensuring we already published a crate that it depends upon.

## TODO

- [ ] Queue a follow-up `[patch]`-removing PR.

consensus/proto_array/Cargo.toml

@@ -10,8 +10,8 @@ path = "src/bin.rs"
 
 [dependencies]
 types = { path = "../types" }
-eth2_ssz = "0.3.0"
-eth2_ssz_derive = "0.2.1"
+eth2_ssz = "0.4.0"
+eth2_ssz_derive = "0.3.0"
 serde = "1.0.116"
 serde_derive = "1.0.116"
 serde_yaml = "0.8.13"

consensus/proto_array/src/proto_array.rs

@@ -1,9 +1,14 @@
 use crate::{error::Error, Block};
 use serde_derive::{Deserialize, Serialize};
+use ssz::four_byte_option_impl;
 use ssz_derive::{Decode, Encode};
 use std::collections::HashMap;
 use types::{AttestationShufflingId, Epoch, Hash256, Slot};
 
+// Define a "legacy" implementation of `Option<usize>` which uses four bytes for encoding the union
+// selector.
+four_byte_option_impl!(four_byte_option_usize, usize);
+
 #[derive(Clone, PartialEq, Debug, Encode, Decode, Serialize, Deserialize)]
 pub struct ProtoNode {
     /// The `slot` is not necessary for `ProtoArray`, it just exists so external components can
@@ -21,11 +26,14 @@ pub struct ProtoNode {
     pub current_epoch_shuffling_id: AttestationShufflingId,
     pub next_epoch_shuffling_id: AttestationShufflingId,
     pub root: Hash256,
+    #[ssz(with = "four_byte_option_usize")]
     pub parent: Option<usize>,
     pub justified_epoch: Epoch,
     pub finalized_epoch: Epoch,
     weight: u64,
+    #[ssz(with = "four_byte_option_usize")]
     best_child: Option<usize>,
+    #[ssz(with = "four_byte_option_usize")]
     best_descendant: Option<usize>,
 }