Add Github CI.

.github/workflows/ci.yml

@@ -0,0 +1,59 @@
+# cribbed from Scenic
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    env:
+      MIX_ENV: test
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - elixir: '1.15.0'
+            otp: '24.2'
+          - elixir: '1.16.0'
+            otp: '26.2'
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - uses: erlef/setup-beam@v1
+      with:
+        otp-version: ${{matrix.otp}}
+        elixir-version: ${{matrix.elixir}}
+
+    - name: Cache build artifacts
+      uses: actions/cache@v3
+      with:
+        path: |
+          ~/.hex
+          ~/.mix
+          _build
+          priv/plts
+        key: ${{ matrix.otp }}-${{ matrix.elixir }}-${{ hashFiles('**/mix.lock') }}
+    - name: Install Dependencies
+      run: |
+        mix local.rebar --force
+        mix local.hex --force
+        mix deps.get
+    - name: Compile Deps
+      run: mix deps.compile
+    - name: Compile
+      run: mix compile --warnings-as-errors
+    - name: Check formatting
+      run: mix format --check-formatted
+      if: startsWith(matrix.elixir, '1.15')
+    - name: Run Tests
+      run: mix test
+#    - name: Run credo
+#      run: mix credo suggest --min-priority=high --ignore-checks Credo.Check.Design.DuplicatedCode