[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
Recommend instead 

```Python
elif f.isFunction or f.is_Mul:
    return univariate_solver(f, symbol)
else:
   ...
```