Update rust edition to 2021

Author: jhg

Cargo.toml

@@ -6,7 +6,7 @@ authors = ["Jesus Hernandez <jesushdez@protonmail.com>"]
 license = "Unlicense"
 keywords = ["pointer", "opaque", "pointers", "cbindgen", "ffi"]
 categories = ["development-tools::ffi", "no-std"]
-edition = "2018"
+edition = "2021"
 readme = "README.md"
 repository = "https://github.com/jhg/opaque-pointer-rs/"
 exclude = [".github/**", ".vscode/**"]