Fix sum and prod functions

[exoRift]

This PR fixes two issues:

subs + sum/prod

console.log(ce.parse('\\sum_{k=0}^{100}k').subs({}).evaluate().toJSON())

Previously, this would error and removing the .subs({}) would fix it.

Now it works.

LaTeX serialization of sum/prod functions

console.log(ce.parse('\\sum_{k=0}^{100}k').toLatex())

Previously, this would log \sum k.

Now, the bounds are properly included