chore: add GitHub CI

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches:
+      - master
+      - 'stage/**'
+  pull_request:
+
+jobs:
+  build:
+    name: ${{ matrix.name || 'Build' }}
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        shell: ${{ matrix.shell || 'sh' }}
+    strategy:
+      matrix:
+        include:
+        - name: Ubuntu
+          os: ubuntu-latest
+        - name: MacOS
+          os: macos-latest
+        - name: Windows
+          os: windows-latest
+          shell: msys2 {0}
+      # complete all jobs
+      fail-fast: false
+    steps:
+    - name: Install MSYS2 (Windows)
+      if: matrix.os == 'windows-latest'
+      uses: msys2/setup-msys2@v2
+      with:
+        path-type: inherit
+        install: curl unzip
+    - name: Install Elan
+      shell: bash -euo pipefail {0}
+      run: |
+        curl -sSfL https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh | sh -s -- -y --default-toolchain none
+        echo "$HOME/.elan/bin" >> $GITHUB_PATH
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Check Lean
+      run: lean --version
+    - name: Build
+      run: lake build
+    - name: Upload Build
+      continue-on-error: true
+      uses: actions/upload-artifact@v2
+      with:
+        name: ${{ matrix.os }}
+        path: build
+    - name: Test
+      run: ./test.sh

Partax/Test.lean

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+import Partax
+import Partax.Test.Basic
+import Partax.Test.LCompile

test.sh

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+#!/usr/bin/env bash
+set -euxo pipefail
+
+${LAKE:-lake} build Partax.Test
+find tests -type f -exec ${LAKE:-lake} env lean {} \;
+echo "all done"