Update ethabi requirement from 16.0 to 18.0

Updates the requirements on [ethabi](https://github.com/rust-ethereum/ethabi) to permit the latest version.
- [Release notes](https://github.com/rust-ethereum/ethabi/releases)
- [Changelog](https://github.com/rust-ethereum/ethabi/blob/master/CHANGELOG.md)
- [Commits](rust-ethereum/ethabi@v16.0.0...v18.0.0)

---
updated-dependencies:
- dependency-name: ethabi
  dependency-type: direct:production
...

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

Author: dependabot[bot]

ethcontract-common/Cargo.toml

@@ -12,7 +12,7 @@ Common types for ethcontract-rs runtime and proc macro.
 """
 
 [dependencies]
-ethabi = "16.0"
+ethabi = "18.0"
 hex = "0.4"
 serde = "1.0"
 serde_derive = "1.0"