Split common crates out into their own repos (#3890)

## Proposed Changes

Split out several crates which now exist in separate repos under `sigp`.

- [`ssz` and `ssz_derive`](https://github.com/sigp/ethereum_ssz)
- [`tree_hash` and `tree_hash_derive`](https://github.com/sigp/tree_hash)
- [`ethereum_hashing`](https://github.com/sigp/ethereum_hashing)
- [`ethereum_serde_utils`](https://github.com/sigp/ethereum_serde_utils)
- [`ssz_types`](https://github.com/sigp/ssz_types)

For the published crates see: https://crates.io/teams/github:sigp:crates-io?sort=recent-updates.

## Additional Info

- [x] Need to work out how to handle versioning. I was hoping to do 1.0 versions of several crates, but if they depend on `ethereum-types 0.x` that is not going to work. EDIT: decided to go with 0.5.x versions.
- [x] Need to port several changes from `tree-states`, `capella`, `eip4844` branches to the external repos.

boot_node/Cargo.toml

@@ -10,7 +10,7 @@ clap = "2.33.3"
 clap_utils = { path = "../common/clap_utils" }
 lighthouse_network = { path = "../beacon_node/lighthouse_network" }
 types = { path = "../consensus/types" }
-eth2_ssz = "0.4.1"
+ethereum_ssz = "0.5.0"
 slog = "2.5.2"
 tokio = "1.14.0"
 log = "0.4.11"