Add HLint action.

.github/workflows/hlint.yml

@@ -0,0 +1,31 @@
+name: HLint CI
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  build:
+    name: hlint
+    runs-on: ubuntu-16.04
+    strategy:
+      matrix:
+        ghc: ["8.8.1"]
+        cabal: ["3.0"]
+
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-haskell@v1
+      name: Setup Haskell
+      with:
+        ghc-version: ${{ matrix.ghc }}
+        cabal-version: ${{ matrix.cabal }}
+
+    - name: Install hlint
+      run: |
+        cabal new-install hlint
+    - name: Run hlint
+      run: |
+        hlint -g

.hlint.yaml

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+- ignore: {name: Avoid lambda}
+- ignore: {name: Eta reduce}
+- ignore: {name: Redundant lambda}
+- ignore: {name: Collapse lambdas}
+- ignore: {name: Use String}
+- ignore: {name: Use putStr, within: Protolude.Show}
+- ignore: {name: Use putStrLn, within: Protolude.Show}
+# CPP preprocessor oddities
+- ignore: {name: Unused LANGUAGE pragma, within: [Protolude.Base, Protolude.CallStack, Protolude.Conv, Protolude.Error]}