handling Irrationals

I'm wondering if the irrational should be parse to some sort of `Variable` type, since currently
this will trigger the `EvalRule` since `is_constant(pi)` etc. is `true` which make rules defined for these irrationals not working for things like `exp` etc.

The issue names is_constant, EvalRule, pi, and exp but gives no files or tests. Start by tracing how irrationals are classified and how EvalRule handles them; clarify whether they should become Variable values, then add coverage showing that rules for functions such as exp apply correctly.

Written by the indexing model from the issue text.