Add GitHub Actions for continuous build/test and creating releases.

.github/workflows/test.yml

@@ -0,0 +1,18 @@
+on: [push, pull_request]
+name: Build/Test
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.16.x
+    - name: Check out code
+      uses: actions/checkout@v2
+    - name: Build
+      run: go build
+    - name: Test
+      run: go test ./...
+    - name: Check formatting
+      run: go fmt ./...