Report memory usage change caused by PRs

On every push to a pull request, the change in memory usage of examples/ConnectionHandlerDemo between the PR's head and base branch for each of the boards in the board matrix will be:
- Shown in the log
- Saved to a workflow artifact
- Commented to the PR's thread as a table (after a delay of ~7 minutes max.)

.github/workflows/compile-examples.yml

@@ -23,8 +23,18 @@ jobs:
       - uses: actions/checkout@v1
         with:
           fetch-depth: 1
+
       - name: Compile examples
         uses: arduino/actions/libraries/compile-examples@master
         with:
           fqbn: ${{ matrix.fqbn }}
           libraries: ${{ env.LIBRARIES }}
+          size-report-sketch: 'ConnectionHandlerDemo'
+          enable-size-deltas-report: 'true'
+
+      - name: Save memory usage change report as artifact
+        if: github.event_name == 'pull_request'
+        uses: actions/upload-artifact@v1
+        with:
+          name: 'size-deltas-reports'
+          path: 'size-deltas-reports'

.github/workflows/report-size-deltas.yml

@@ -0,0 +1,13 @@
+name: Report PR Size Deltas
+
+on:
+  schedule:
+    - cron:  '*/5 * * * *'
+
+jobs:
+  report:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Comment size deltas reports to PRs
+        uses: arduino/actions/libraries/report-size-deltas@master