Remove recursive evaluation

[Specy]

Many many places use recursion to solve the expressions, which is not the best in the case of unrolling a sum. Try to either use a dynamic alloc stack or remove the recursion Altogether

[Specy]

Or... alternatively for now:
https://github.com/rust-lang/stacker

[Specy]

Or... alternatively for now: https://github.com/rust-lang/stacker

this works but, there needs to find a way to drop the PreExp and Exp iteratively instead of recursively as it stack overflows on drop