Feature/mongo 2.6.5 expressions Let

Ordered functions to match the c++ code.  Minor grammar changes 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. ;-)
ok. The javascript and c++ seem to agree in this function.