Add a basic CI workflow

Just a simple `cargo check`, `cargo fmt --all -- --check`, and `sudo
cargo test`.

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+---
+name: Rust CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo check
+  fmt:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo fmt --all -- --check
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo build --tests
+      - run: sudo -E .cargo/bin/cargo test