Fix the Rust interpreter for the new syntax about bril

[sampsyo]

A good way to go about this might be to refactor the interpreter to use the new Rust library instead of its own deserializer.

[yati-sagade]

Thanks for making the course! This was a nice task to get familiar with bril. I've left out call for a follow-up, since it wasn't implemented here anyway, and it would likely require some additional changes looking at the TypeScript implementation. I've also left out all the extensions except float, which was simple enough to implement.