[WIP] Rewriting the Univariate Solver

This PR aims to rewrite a cleaner and robust univariate equation solver.
- [x] polynomial
- [x] rational
- [x] trigonometric
- [ ] functions solvable by LambertW
- [ ] write 4 tests to complete coverage
- [ ] correct typo
I'll send a PR:

```
>>> solveset(x-2<0,x)
sympy\solvers\solve_univariate.py:843: UserWarning:
The variable you are solving for is complex but will assumed to be
real since solving complex inequalities is not supported.
  '''))
(-oo, 2)
>>> solveset(x-2<3,x)
(-oo, 5)
```