This is a follow up ticket to SERVER-41065, where we thought that it would make more sense to make sense to remove 'Variables' object from 'ExpressionContext'. Since 'evaluate()' uses the variables passed as the parameters, there is no value in having them in 'ExpressionContext'

As part of this ticket we can also make ExpressionContext const through out the expression tree. 'ExpressionContext' is not being modified anymore and this change should be straight forward.