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](https://github.com/paritytech/parity-common/compare/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] <[email protected]>