Bump ethereum_ssz_derive from 0.5.4 to 0.8.1

Bumps [ethereum_ssz_derive](https://github.com/sigp/ethereum_ssz) from 0.5.4 to 0.8.1.
- [Release notes](https://github.com/sigp/ethereum_ssz/releases)
- [Commits](sigp/ethereum_ssz@v0.5.4...v0.8.1)

---
updated-dependencies:
- dependency-name: ethereum_ssz_derive
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Cargo.lock

@@ -42,7 +42,7 @@ client = { git = "https://github.com/ackintosh/lighthouse.git", rev = "b668e5883
 environment = { git = "https://github.com/ackintosh/lighthouse.git", rev = "b668e5883dd03e6667bd5976d60e12b8580fab7f" }
 eth2_network_config = { git = "https://github.com/ackintosh/lighthouse.git", rev = "b668e5883dd03e6667bd5976d60e12b8580fab7f" }
 ethereum_ssz = "0.5.4"
-ethereum_ssz_derive = "0.5.4"
+ethereum_ssz_derive = "0.8.1"
 slot_clock = { git = "https://github.com/ackintosh/lighthouse", rev = "b668e5883dd03e6667bd5976d60e12b8580fab7f" }
 types = { git = "https://github.com/ackintosh/lighthouse.git", rev = "b668e5883dd03e6667bd5976d60e12b8580fab7f" }