Add no_std to CI (dalek-cryptography#289)

* Add no_std to CI

* Add serde to no_std feature test

* Try out cargo hack

* No serde - expect success

* Add build for no-default-features

* Exclude default

.github/workflows/rust.yml

@@ -70,6 +70,21 @@ jobs:
     - uses: dtolnay/[email protected]
     - run: cargo build
 
+  build-nostd:
+    name: Build on no_std target (thumbv7em-none-eabi)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: stable
+          targets: thumbv7em-none-eabi
+      - uses: taiki-e/install-action@cargo-hack
+      # No default features build
+      - run: cargo build --target thumbv7em-none-eabi --release --no-default-features
+      # TODO: serde pending PR#288
+      - run: cargo hack build --target thumbv7em-none-eabi --release --each-feature --exclude-features default,std,serde
+
   bench:
     name: Check that benchmarks compile
     runs-on: ubuntu-latest