Avoid risky JSON.stringify in utils.IIFE.

This helps with issues like https://github.com/benjamn/recast/issues/135, because `Lines` objects are not (currently) `JSON.stringify`-serializable.

I also took the liberty of changing the name from `IFFE` to `IIFE`, since it creates an Immediately-Invoked Function Expression ;)

Unrelated: http://www.beatport.com/release/iffy/1386702