LET expression - Ordered functions to match the c++ code.  Minor grammar ch...

...anges only.

Made this a subclass of FixedArityExpression.

Changing the order of the functions may help someone in the future when they diff the code against the c++.   But I am not sure it was helpful for us, now. ;-)
In mongo 2.6.5, the addDependencies() function has changed to accept a DepsTracker as its first argument.  ObjectExpression and a few others also use this.  This will be in Dependencies.js, which Austin is working on.