Enable GitHub Actions

.github/workflows/ci.yml

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+name: test
+
+on:
+  push:
+  pull_request:
+  schedule: [cron: "40 1 * * *"]
+
+jobs:
+  test:
+    name: Rust ${{matrix.rust}} on ${{matrix.os}}
+    runs-on: ${{matrix.os}}-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        rust: [nightly, beta, stable, 1.31.0]
+        os: [ubuntu, macos, windows]
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{matrix.rust}}
+      - run: cargo test