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

@@ -247,3 +247,19 @@ jobs:
           rustup component add rustfmt
       - name: Run rustfmt checks
         run: ci/rustfmt.sh
+
+  incremental-mutants:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: Relative diff
+        run: |
+          git branch -av
+          git diff origin/main.. | tee git.diff
+      - uses: Swatinem/rust-cache@v2
+      - name: Mutants
+        run: |
+          cargo install cargo-mutants
+          cargo mutants --no-shuffle -j 2 -vV --in-diff git.diff