[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
side note: is there some `evaluate=False` convention in the set module?  Just to keep auto-evaluation from doing all such things.  @mrocklin ?