Add CI

.github/workflows/ci.yml

+name: CI
+
+# Trigger the workflow on push or pull request, but only for the master branch
+on:
+  pull_request:
+  push:
+    branches: [master]
+
+jobs:
+  cabal:
+    name: ${{ matrix.os }} / ghc ${{ matrix.ghc }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest]
+        cabal: ["latest"]
+        ghc: ["8.6", "8.8", "8.10"]
+
+    steps:
+    - uses: actions/checkout@v2
+      if: github.event.action == 'opened' || github.event.action == 'synchronize' || github.event.ref == 'refs/heads/master'
+
+    - uses: actions/setup-haskell@v1.1
+      id: setup-haskell-cabal
+      name: Setup Haskell
+      with:
+        ghc-version: ${{ matrix.ghc }}
+        cabal-version: ${{ matrix.cabal }}
+
+    - name: Build
+      run: |
+        cabal update
+        cabal build all --enable-tests --enable-benchmarks --write-ghc-environment-files=always
+
+    - name: Test
+      run: |
+        cabal test all --enable-tests