ci: add GitHub Actions CI.

.github/workflows/coverage.yaml

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+name: coverage
+
+on: [push, pull_request]
+
+jobs:
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Set up Go
+      uses: actions/setup-go@v4
+      with:
+        go-version: "1.21"
+
+    - name: Test
+      run: go test -v ./... -coverprofile=coverage.out
+
+    - name: Upload coverage report
+      uses: codecov/codecov-action@v3
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}

.github/workflows/test.yaml

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+name: test
+
+on: [push, pull_request]
+
+jobs:
+
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        go_version: ["1.19", "1.20", "1.21"]
+        os: [ubuntu-latest, windows-latest, macOS-latest]
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Set up Go ${{ matrix.go_version }}
+      uses: actions/setup-go@v4
+      with:
+        go-version: ${{ matrix.go_version }}
+
+    - name: Test
+      run: go test -v ./...