CircleCI→GitHub Actions (#740)

* CircleCI→GitHub Actions
* Fix gofmt files

.github/workflows/CI.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    types: [opened, reopened, synchronize]
+
+jobs:
+  build-test:
+    name: Build, test & format
+    strategy:
+      matrix:
+        go-version: [1.15.x, 1.16.x]
+        platform: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.platform }}
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: setup go
+      uses: actions/setup-go@v2
+      with:
+        go-version: ${{ matrix.go-version }}
+
+    - name: Build
+      run: go build -v ./...
+
+    - name: Test
+      run: go test -v ./...
+
+    - name: Format
+      if: matrix.platform == 'ubuntu-latest'
+      run: if [ "$(gofmt -s -l . | wc -l)" -gt 0 ]; then exit 1; fi