Add string-table check in github actions

This was formerly a Travis CI job. Setting it up using Github actions
gives us a user interface which integrates into the rest of github
better. This is a step towards having all jobs seup in Github actions
and removing Travis.

Author: thomasspriggs

.github/workflows/pull-request-checks.yaml

@@ -164,3 +164,10 @@ jobs:
           BASE_BRANCH: ${{ github.base_ref }}
           MERGE_BRANCH: ${{ github.ref }}
         run: ./.github/workflows/pull-request-check-cpplint.sh
+
+  check-string-table:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check for unused irep ids
+        run: ./scripts/string_table_check.sh