CI: test with cargo-careful

`cargo careful` is a tool to run your Rust code extra carefully --
opting into a bunch of nightly-only extra checks that help detect
Undefined Behavior, and using a standard library with debug assertions.

https://github.com/RalfJung/cargo-careful

.github/workflows/hybrid-array.yml

@@ -68,6 +68,14 @@ jobs:
       - run: rustup component add miri && cargo miri setup
       - run: cargo miri test --target ${{ matrix.target }} --all-features
 
+  careful:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+      - run: cargo install cargo-careful
+      - run: cargo careful test --all-features
+
   clippy:
     runs-on: ubuntu-latest
     steps: