Add wasm-bindgen.

src/bin_wasm.rs

@@ -1,6 +1,7 @@
 #![no_main]
+use wasm_bindgen::prelude::wasm_bindgen;
 
-#[no_mangle]
-pub extern "C" fn add(left: usize, right: usize) -> usize {
+#[wasm_bindgen]
+pub fn add(left: usize, right: usize) -> usize {
     left + right
 }