Upgrade to rust 2021.

Cargo.toml

@@ -3,7 +3,7 @@ name = "labrpc"
 version = "0.2.1"
 authors = ["Jing Yang <ditsing@gmail.com>"]
 description = "A RPC framework for testing"
-edition = "2018"
+edition = "2021"
 license = "MIT"
 readme = "README.md"
 repository = "https://github.com/ditsing/labrpc"