Implement the Racket number pyramid

Racket numbers are exact, in-exact, complex and can be arbitrarily large as opposed to 64-bit floats in JavaScript. 

- Whalesong and Pyret use Danny Yoo's [`js-numbers`](https://github.com/brownplt/pyret-lang/blob/master/src/js/base/js-numbers.js) library. IIRC the library itself models the number pyramid, and hence can be used directly.
