Bump impl-serde from 0.3.2 to 0.4.0

Bumps [impl-serde](https://github.com/paritytech/parity-common) from 0.3.2 to 0.4.0.
- [Release notes](https://github.com/paritytech/parity-common/releases)
- [Commits](paritytech/parity-common@impl-serde-v0.3.2...impl-serde-v0.4.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

primitives/chain-millau/Cargo.toml

@@ -16,7 +16,7 @@ bp-runtime = { path = "../runtime", default-features = false }
 fixed-hash = { version = "0.7.0", default-features = false }
 hash256-std-hasher = { version = "0.15.2", default-features = false }
 impl-codec = { version = "0.6", default-features = false }
-impl-serde = { version = "0.3.1", optional = true }
+impl-serde = { version = "0.4.0", optional = true }
 parity-util-mem = { version = "0.12", default-features = false, features = ["primitive-types"] }
 scale-info = { version = "2.1.1", default-features = false, features = ["derive"] }
 serde = { version = "1.0", optional = true, features = ["derive"] }