Improvements to inequality solving

fixes #8206 (please feel free to any others that are related that also work with these changes)
fixes #8518
If we want to make this more general we have to (for now) accept that solutions will not be complete (e.g. solve(sin(x) > 1/2) will only give a subset of solutions). In addition, if we go with the more general approach then using the singularities routine would not be appropriate since that's only for rational functions. I can make the change and you can see what you think.

> slow review

No problem. Thanks for looking it over.