Introduce basic incremental mutation testing

We introduce a CI job for mutation testing of PR diffs using cargo-mutants.
Missed cases do not trigger a fail of this job yet as we just introduce it
now for visibility. We may start enforcing stricter rules at a later stage.

Author: dunxen

.github/workflows/build.yml

@@ -216,3 +216,26 @@ jobs:
       - name: Run default clippy linting
         run: |
           cargo clippy -- -Aclippy::erasing_op -Aclippy::never_loop -Aclippy::if_same_then_else -Dclippy::try_err
+
+  mutation:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: Relative diff
+        run: |
+          git branch -av
+          git diff origin/${{ github.base_ref }}.. | tee git.diff
+      - run: cargo install cargo-mutants
+      - name: Mutants
+        run: |
+          cargo mutants --no-shuffle -j 2 -vV --in-diff git.diff
+      - name: Archive mutants.out
+        uses: actions/upload-artifact@v3
+        if: always()
+        with:
+          name: mutants-incremental.out
+          path: mutants.out