Having multiple kinds of node that hold numeric values made things more
complicated than they needed to be, and we were already converting
ConstantCalculationNodes to NumericCalculationNodes in the first
simplification pass that happens at parse-time, so they didn't exist
after that.
As noted, the spec allows for other contexts to introduce their own
numeric keywords, which might be resolved later than parse-time. We'll
need a different mechanism to support those, but
ConstantCalculationNode could not have done so anyway.