add tokio example

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

examples/tokio/wasm/tokio-wasi.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let name = std::env::var("NAME").unwrap();
+    println!("Hello, world! My name is {}", name);
+    std::thread::sleep(std::time::Duration::from_secs(1));
+    println!("Goodbye from {}", name);
+}