Symbol support for procedural macro functions

Things like [javascript iterables require assigning a function to a symbol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/asyncIterator#user-defined_async_iterables). It would be ideal if this were a thing that the `node-bindgen` proc macro could help with.

I'm sure there are other cases where we will want this as well.