Add tokio ^0.2 support (#440)

* Use fs::write for writing files
* Fix running tests without tokio

.github/workflows/test.yml

@@ -39,6 +39,10 @@ jobs:
         with:
           command: test
           args: --features=async-std1
+      - uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --features=tokio02
 
   check:
     name: Check