Map JS Types to Rust

Currently we only support passing `i32` integers — great for math, but having more options would be great! We should probably look into [rust-experiments](https://github.com/killercup/wasm-experiments) and add some of the things they're doing.

Also s/o to @killercup for giving pointers to get in the right direction! 🙏

## See Also
- https://www.reddit.com/r/rust/comments/7fqvif/rust_webassembly_transform_for_browserify/
- https://github.com/killercup/wasm-experiments