migrate CI to GitHub Actions

use codecov.io to report on code coverage.
fix a few issues raised by golangci-lint

.github/workflows/linter.yaml

@@ -0,0 +1,18 @@
+on: [push, pull_request]
+name: linter
+
+jobs:
+  lint:
+    strategy:
+      matrix:
+        go-version: [1.x]
+        platform: [ubuntu-latest]
+    runs-on: ${{ matrix.platform }}
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: golangci-lint
+      uses: golangci/golangci-lint-action@v2
+      with:
+        version: v1.41