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