Try cargo-mutants in CI

Author: sourcefrog

.github/workflows/mutants.yaml

@@ -0,0 +1,20 @@
+name: cargo-mutants
+
+on: [pull_request, push]
+
+jobs:
+  mutation:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+      - name: Install cargo-mutants
+        run: cargo install cargo-mutants
+      - name: Run mutant tests
+        run: cargo mutants -- --all-features
+      - name: Archive results
+        uses: actions/upload-artifact@v3
+        if: failure()
+        with:
+          name: mutation-report
+          path: mutants.out