Preserving type with Num(::Irrational)

[moble]

As described on discourse, using irrational numbers with symbolic expressions frequently doesn't work as expected. Specifically, even if we ask for the irrational to be converted to a Num, it will get converted to a Float64 as soon as it is actually used in an algebraic expression. We need to be able to preserve types, including retaining correct behavior even after the expression is used in ModelingToolkit.

I also found some discussion of this problem in #496, and in this older discourse post.