Add a check that the code is formatted (#10)

This will prevent merge conflicts in future PRs

.github/workflows/check.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+cargo fmt --check || (
+    echo
+    echo "Please run 'cargo fmt' to format the code."
+    exit 1
+)

.github/workflows/check.yml

@@ -0,0 +1,25 @@
+name: Check code
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Rust Cache
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            target
+          key: ${{ runner.os }}-rust-release-v5-${{ hashFiles('Cargo.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-rust-release-v5
+            ${{ runner.os }}-rust-release
+
+      - name: Run checks
+        run: .github/workflows/check.sh