Add GitHub workflow: Rust formatting.

Author: gendx

.github/workflows/format.yml

@@ -0,0 +1,14 @@
+on: [push, pull_request]
+name: Formatting
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    env:
+      CARGO_UNSTABLE_SPARSE_REGISTRY: true
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@nightly
+        with:
+          components: rustfmt
+      - name: Check Rust formatting
+        run: cargo fmt --verbose --manifest-path src/android-simd/Cargo.toml -- --check