Remove call to `eval` in `simplify`

I have some code where I want to use Calculus.jl to differentiate symbolic expressions inside a `@generated` function. This currently does not work because after differentiating Calculus.jl calls `simplify`, which in turn often ends up calling `eval`. As of right now `@generated` functions are not allowed to call eval.

I'm opening this issue to see if those who know the internals here better think it would be possible/feasible to remove the call to `eval` inside `simplify`?

ref: https://github.com/EconForge/Dolang/pull/31#issuecomment-307791343