CI: test with no-default-features and check doc

This is used for no-std environments.

Author: fengalin

.github/workflows/CI.yml

@@ -27,5 +27,9 @@ jobs:
     - uses: actions/checkout@v2
     - name: Build
       run: cargo build --verbose
-    - name: Tests
+    - name: Tests default features
       run: cargo test --verbose
+    - name: Tests no default features
+      run: cargo test --verbose --no-default-features
+    - name: Check doc generation
+      run: cargo doc --verbose