Update strum requirement from 0.21.0 to 0.22.0

Updates the requirements on [strum](https://github.com/Peternator7/strum) to permit the latest version.
- [Release notes](https://github.com/Peternator7/strum/releases)
- [Changelog](https://github.com/Peternator7/strum/blob/master/CHANGELOG.md)
- [Commits](https://github.com/Peternator7/strum/commits)

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

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

chain-impl-mockchain/Cargo.toml

@@ -17,7 +17,7 @@ typed-bytes = { path = "../typed-bytes" }
 rand_core = "0.6"
 imhamt = { path = "../imhamt" }
 sparse-array = { path = "../sparse-array" }
-strum = "0.21.0"
+strum = "0.22.0"
 strum_macros = "0.21.0"
 hex = { version = "0.4.2", default-features = false, features = [ "std" ] }
 quickcheck = { version = "0.9", optional = true }