Rewrite in Rust & webassembly

[dignifiedquire]

After reading https://hacks.mozilla.org/2018/01/oxidizing-source-maps-with-rust-and-webassembly/ I think we are ready to move this forward. Maintaining a cbor parser in rust will be much less painful than hand rolled asm.js and potentially allow us to move even more logic into rust/webassembly.