Upgrade to edition 2021, set MSRV

I used `cargo msrv -- cargo test` tool to find that 1.56.1 is the oldest satisfied MSRV

.github/workflows/rust.yml

@@ -13,7 +13,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Build
       run: cargo build --verbose
     - name: Run tests
@@ -22,13 +22,19 @@ jobs:
   msrv:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
-    - name: Install MSRV version
-      run: cargo install
+    - uses: actions/checkout@v4
+    - name: Read crate metadata
+      id: metadata
+      run: echo "rust-version=$(sed -ne 's/rust-version *= *\"\(.*\)\"/\1/p' Cargo.toml)" >> $GITHUB_OUTPUT
+    - name: Install Rust
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        toolchain: ${{ steps.metadata.outputs.rust-version }}
+    - run: cargo test --verbose
 
   fmt:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Build
       run: cargo fmt --check

Cargo.toml

@@ -9,6 +9,7 @@ repository = "https://github.com/servo/rust-fnv"
 documentation = "https://doc.servo.org/fnv/"
 categories = ["algorithms", "data-structures"]
 edition = "2021"
+rust-version = "1.56.1"
 
 [lib]
 name = "fnv"