Run mutants in CI after other tests pass (#147)

- Fold MSRV into a single matrix and test nightly
- Run tests on every push
- Install rustfmt
- Run cargo-mutants only after tests pass
- Don't need to test install in each test
- Use rust-toolchain for mutants too

.github/workflows/tests.yml

@@ -3,11 +3,7 @@ name: Tests
 permissions:
   contents: read
 
-on:
-  push:
-    branches:
-      - main
-  pull_request:
+on: push
 
 # see https://matklad.github.io/2021/09/04/fast-rust-builds.html
 env:
@@ -25,11 +21,16 @@ jobs:
     strategy:
       matrix:
         os: [macOS-latest, ubuntu-latest, windows-latest]
+        version: [stable, nightly, "1.70"]
 
     runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.version }}
+          components: rustfmt
       - name: Show Cargo and rustc version
         run: |
           cargo --version
@@ -41,5 +42,23 @@ jobs:
         run: cargo build --all-targets
       - name: Test
         run: cargo test --workspace
-      - name: Install
-        run: cargo install --path .
+
+  cargo-mutants:
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: beta
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo install --path .
+      - name: Mutants
+        run: |
+          cargo mutants --no-shuffle --exclude console.rs -j 2
+      - name: Archive mutants.out
+        uses: actions/upload-artifact@v3
+        if: always()
+        with:
+          name: mutants.out
+          path: mutants.out