Add CI to try building under no_std targets.

.github/workflows/default.yml

@@ -20,6 +20,14 @@ jobs:
       run: cargo build --verbose
     - name: Run tests
       run: cargo test --verbose
+    - name: Check under no_std
+      run: |
+        # thumbv6m-none-eabi is a Tier 2 target which has no `std` and no atomic pointers.
+        rustup target add thumbv6m-none-eabi
+        cargo check --verbose --target thumbv6m-none-eabi
+        # aarch64-unknown-none is Tier 2, has no `std`, but does have atomic pointers.
+        rustup target add aarch64-unknown-none
+        cargo check --verbose --target aarch64-unknown-none
     
   miri-test:
     runs-on: ubuntu-latest