Add Cargo.lock to the repo to fix flaky CI

Previously, CI was breaking "randomly" every time a dependency moved its MSRV beyond whatever is used to run in CI. To fix that, just commit the lockfile to git so that CI runs with a fixed set of dependencies. Also adds my own `rust-cache` for good measure :-)

.github/workflows/ci.yml

@@ -7,10 +7,11 @@ jobs:
     name: Test
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
-      - uses: dtolnay/rust-toolchain@1.60.0
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@1.72.0
         with:
           components: rustfmt
+      - uses: Swatinem/rust-cache@v2
       - run: cargo fmt -- --check
       - run: cargo test
       - run: cargo bench

.gitignore

@@ -1,2 +1 @@
 target
-Cargo.lock