Update to 2021 edition

Author: dtolnay

Cargo.toml

@@ -5,7 +5,7 @@ authors = ["David Tolnay <[email protected]>"]
 categories = ["rust-patterns", "no-std", "no-std::no-alloc"]
 description = "Make trait methods callable without the trait in scope"
 documentation = "https://docs.rs/inherent"
-edition = "2018"
+edition = "2021"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/inherent"
 rust-version = "1.56"