Explained some of what currently takes place within lambda-free.fun

Specifically, I commented on the newScope function with a summary of its purpose,
and on the Fun branch of the exp function with an explanation of individual steps.
Merge branch 'master' of https://github.com/DanRosenwasser/mlton
Merge pull request #8 from DanRosenwasser/master

Commented lambda-free.fun

The code in lambda-free.fun utilizes mutability somewhat heavily, and uses fairly clever ways of accomplishing tasks in few steps. Unfortunately this makes it difficult to understand at a first glance. This minor changeset should provide some basic idea on how to approach/maintain the code contained in lambda-free.fun in the future.