Emit the difference between fast.js and build in method

Fixes #34.
Sadly we can't assume that the first method is a builtin, consider [bench/clone.js](https://github.com/codemix/fast.js/blob/master/bench/clone.js) which has no builtin to compare against, instead it compares against lodash and underscore. We need a cleverer way to mark native functions, even if that's just always prefixing their names with `Native`, so `"Native Array::map()"` instead of `"Array::map()"`