Integrate GitHub Actions.

- Automatically build with Go 1.12.
- Check for stale issues.

.gitlab/workflows/go.yml

@@ -0,0 +1,22 @@
+name: Go
+
+on: [push]
+
+jobs:
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: Set up Go 1.12
+        uses: actions/setup-go@v1
+        with:
+          go-version: 1.12
+        id: go
+
+      - name: Check out code into the Go module directory
+        uses: actions/checkout@v1
+
+      - name: Build
+        run: go build -v .

.gitlab/workflows/stale.yml

@@ -0,0 +1,19 @@
+name: Mark stale issues and pull requests
+
+on:
+  schedule:
+    - cron: "0 0 * * *"
+
+jobs:
+  stale:
+
+    runs-on: ubuntu-latest
+    
+    steps:
+      - uses: actions/stale@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          stale-issue-message: 'Stale issue message'
+          stale-pr-message: 'Stale pull request message'
+          stale-issue-label: 'no-issue-activity'
+          stale-pr-label: 'no-pr-activity'