Update rust edition to 2021

Author: mpizenberg

Cargo.toml

@@ -2,7 +2,7 @@
 name = "elm-test-rs"
 version = "2.0.0"
 authors = ["Matthieu Pizenberg <matthieu.pizenberg@gmail.com>", "Harry Sarson <harry.sarson@hotmail.co.uk>"]
-edition = "2018"
+edition = "2021"
 description = "Simple and fast Rust alternative to node-test-runner to run elm tests"
 license = "BSD-3-Clause"