Fix the wasm2js example (#1750)

This needed and update now that we're explicitly importing `*.wasm` to
import `*.js` instead. Additionally this was moved over to the `web`
target to avoid needing Webpack

Closes #1743

examples/wasm2js/src/lib.rs

@@ -7,7 +7,7 @@ extern "C" {
     fn log(s: &str);
 }
 
-#[wasm_bindgen]
+#[wasm_bindgen(start)]
 pub fn run() {
     log("Hello, World!");
 }