Update strum_macros requirement from 0.18.0 to 0.19.1

Updates the requirements on [strum_macros](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)

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

chain-impl-mockchain/Cargo.toml

@@ -16,7 +16,7 @@ rand_core = "0.5"
 imhamt = { path = "../imhamt" }
 sparse-array = { path = "../sparse-array" }
 strum = "0.18.0"
-strum_macros = "0.18.0"
+strum_macros = "0.19.1"
 cfg-if = "0.1"
 hex = { version = "0.4.2", default-features = false, features = [ "std" ] }
 quickcheck = { version = "0.9", optional = true }