feat: add example4

__wasm/wasmtime/wasms/simple_rust_wasm/src/lib.rs

@@ -0,0 +1,12 @@
+#[link(wasm_import_module = "host")]
+extern "C" {
+
+     #[link_name = "hello"]
+    fn host_hello(a: i32);
+}
+
+#[no_mangle]
+pub extern "C" fn hello() {
+  unsafe { host_hello(3); }
+}
+