Add a CI job to verify formatting

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
             { chip: esp32s3, features: "direct-boot,eh1,smartled,ufmt" },
           ]
     env:
-      RUSTFLAGS: "--cfg target_has_atomic=\"8\" --cfg target_has_atomic=\"16\" --cfg target_has_atomic=\"32\" --cfg target_has_atomic=\"ptr\""
+      RUSTFLAGS: '--cfg target_has_atomic="8" --cfg target_has_atomic="16" --cfg target_has_atomic="32" --cfg target_has_atomic="ptr"'
     steps:
       - uses: actions/checkout@v2
       - uses: esp-rs/xtensa-toolchain@v1.4
@@ -71,7 +71,36 @@ jobs:
           args: -Zbuild-std=core --examples --manifest-path=${{ matrix.chip_features.chip }}-hal/Cargo.toml --target=xtensa-${{ matrix.chip_features.chip }}-none-elf --features=${{ matrix.chip_features.features }}
 
   # --------------------------------------------------------------------------
-  # Clippy
+  # Formatting & Clippy
+
+  rustfmt:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        package:
+          [
+            esp-hal-common,
+            esp32-hal,
+            esp32c2-hal,
+            esp32c3-hal,
+            esp32s2-hal,
+            esp32s3-hal,
+          ]
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          override: true
+          components: rustfmt
+      - uses: Swatinem/rust-cache@v1
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all --manifest-path=${{ matrix.package }}/Cargo.toml -- --check
 
   clippy-riscv:
     name: Run clippy on RISC-V builds