Update to Rust 2021 edition

Author: dtolnay

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.31.0]
+        rust: [nightly, beta, stable, 1.56.0]
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@master

Cargo.toml

@@ -8,8 +8,8 @@ repository = "https://github.com/serde-rs/example-format"
 documentation = "https://serde.rs/data-format.html"
 keywords = ["serde"]
 categories = ["encoding"]
-edition = "2018"
-rust-version = "1.31"
+edition = "2021"
+rust-version = "1.56"
 publish = false
 
 [dependencies]