CI: Add GitHub Actions

.github/workflows/ci.yml

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+# Adapted from https://kodimensional.dev/github-actions
+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:
+      matrix:
+        # os: [ubuntu-latest, macOS-latest, windows-latest]
+        os: [ubuntu-latest]
+        cabal: ["3.2"]
+        ghc:
+          - "8.6.5"
+          - "8.8.4"
+          - "8.10.4"
+
+    steps:
+    - uses: actions/checkout@v2
+      if: github.event.action == 'opened' || github.event.action == 'synchronize' || github.event.ref == 'refs/heads/master'
+
+    - uses: haskell/actions/setup@v1
+      id: setup-haskell-cabal
+      name: Setup Haskell
+      with:
+        ghc-version: ${{ matrix.ghc }}
+        cabal-version: ${{ matrix.cabal }}
+
+    - name: Freeze
+      run: |
+        cabal freeze
+
+    - uses: actions/cache@v1
+      name: Cache ~/.cabal/store
+      with:
+        path: ${{ steps.setup-haskell-cabal.outputs.cabal-store }}
+        key: ${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('cabal.project.freeze') }}
+
+    - name: Build
+      run: |
+        cabal configure --enable-tests --enable-benchmarks --test-show-details=direct --write-ghc-environment-files=always
+        cabal build all
+
+    - name: Test
+      run: |
+        cabal test all
+
+  stack:
+    name: stack / ghc ${{ matrix.ghc }}
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        # stack: ["2.3.1"]
+        stack: ["latest"]
+        ghc: ["8.10.4"]
+
+    steps:
+    - uses: actions/checkout@v2
+      if: github.event.action == 'opened' || github.event.action == 'synchronize' || github.event.ref == 'refs/heads/master'
+
+    - uses: haskell/actions/setup@v1
+      name: Setup Haskell Stack
+      with:
+        ghc-version: ${{ matrix.ghc }}
+        enable-stack: true
+        stack-version: ${{ matrix.stack }}
+
+    - uses: actions/cache@v1
+      name: Cache ~/.stack
+      with:
+        path: ~/.stack
+        key: ${{ runner.os }}-${{ matrix.ghc }}-stack
+
+    - name: Build
+      run: |
+        stack build --system-ghc --test --bench --no-run-tests --no-run-benchmarks
+
+    - name: Test
+      run: |
+        stack test --system-ghc