[WIP] Rewriting the Univariate Solver

This PR aims to rewrite a cleaner and robust univariate equation solver.
- [ ] Univariate
    - [x] polynomial
    - [x] rational
    - [ ] trigonometric
    - [ ] functions with exp and log
- [ ] Multivariate
> The idea was to start from the leafs of the expression tree, not from the top.

How does the direction of the traversal matters, if we are that we test for every subexpression.