Add a CI check for code formatting (#151)

This PR adds a GitHub action (🎉) that checks for the latest refmt syntax.

This is done thanks to @thomsj which brought up the problems with formatting in the repo! 👏 #105 (comment)

The current implementation is not ideal: it installs esy and then installs OCaml and @esy-ocaml/reason. A better way to do it is to distribute refmt as a standalone binary from the latest Reason release. This will be fast to install and therefore fast to integrate with CI.

An approach to do so is to add an artifact to the CircleCI runs, and then use something like circleci-artifacts.now.sh to fetch the artifact, extract it and profit 💰

In the meantime, it takes <4m to run it, which is much faster than the Azure Linux build, not to mention the Windows one.

.github/workflows/refmt.yml

@@ -0,0 +1,63 @@
+name: Check formatting
+on: [push]
+
+# consider using docker: ocaml/opam2:4.06
+# - sudo apt-get update && apt-get install -y m4
+# - opam update && opam install reason
+# - refmt # is new
+
+jobs:
+  # build:
+  #   name: build project
+  #   runs-on: ubuntu-latest
+  #   steps:
+  #     - uses: actions/checkout@master
+  #       with:
+  #         fetch-depth: 1
+  #     - uses: actions/setup-node@v1
+  #       with:
+  #         node-version: '10.x'
+  #     - run: npm install -g esy@latest
+  #     - run: esy i
+  #     - run: esy b
+  #     - run: esy export-dependencies
+  #     - run: tar -czf build.tar.gz _export
+  #     - uses: actions/upload-artifact@master
+  #       with:
+  #         name: cache-linux-build-dependencies
+  #         path: build.tar.gz
+
+  test_refmt:
+    name: run refmt
+    runs-on: ubuntu-latest
+    # needs: [build]
+    steps:
+      - uses: actions/checkout@master
+        with:
+          fetch-depth: 1
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '10.x'
+      # - run: npm install -g esy@latest
+      # - uses: actions/download-artifact@master
+      #   with:
+      #     name: cache-linux-build-dependencies
+      # - run: tar -xzf cache-linux-build-dependencies/build.tar.gz
+      # - run: esy import-build _export/*
+      # - run: esy i
+      # - run: esy fmt
+      - run: npm i -g esy@latest
+      - run: esy @fmt i
+      - run: esy @fmt refmt --in-place */*.re
+      # - run: ./scripts/fmt_all.sh
+      #   name: refmt all the files
+      - run: |
+          if [ -n "$(git status --porcelain)" ]; then
+            echo "There are differences:"
+            git status --porcelain
+            echo "----"
+            echo ""
+            echo 'Please run `esy fmt` locally to fix the issues.'
+            exit 1
+          fi
+        name: "Throw if something changed"

fmt.esy.lock/.gitattributes

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+
+# Set eol to LF so files aren't converted to CRLF-eol on Windows.
+* text eol=lf

fmt.esy.lock/.gitignore

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+
+# Reset any possible .gitignore, we want all esy.lock to be un-ignored.
+!*