Bump edition from 2018 to 2021.

Doesn't require any code changes.

Author: nnethercote

Cargo.toml

@@ -7,7 +7,7 @@ license = "Apache-2.0/MIT"
 readme = "README.md"
 keywords = ["hash", "hasher", "fxhash", "rustc"]
 repository = "https://github.com/rust-lang/rustc-hash"
-edition = "2018"
+edition = "2021"
 
 [features]
 default = ["std"]