Implement CI using GitHub Actions

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: Cross-platform build & test
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+          - windows-latest
+        rust:
+          - stable
+          - beta
+          - nightly
+    runs-on: ${{ matrix.os }}
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build
+      run: cargo build --all --locked --verbose
+    - name: Run tests
+      run: cargo test --all --locked --verbose
+    - name: Check code formatting
+      run: cargo fmt --all -- --check