[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
> Or more precisely
> 
> ```Python
> In [3]: x in S.Reals 
> Out[3]: True
> ```

It should return `None` just like `x.is_real`.