Add missing wasm_bindgen for Universe implementation

[ramonberrutti]

Summary

Following the tutorial, I found that the first implementation of the Universe structure is missing the macro of wasm_bindgen. I know that in the following code is set, but when you are following a tutorial, you only focus on parts that change.

[elasticdotventures]

See issue # (https://github.com/rustwasm/book/issues/211)