First Order Logic

- [ ] Make sure tests pass
- [x] Use Symbol everywhere, not strings (or Constant or Variable or whatever)
- [ ] Add docstring with doctest to every class and function (including and especially abstract classes, although the abstract classes do not need to have doctests if that does not make sense). Major methods of classes should have this as well. 
- [x] Address https://github.com/sympy/sympy/pull/7608#discussion_r15020433
- [x] Address https://github.com/sympy/sympy/pull/7608#discussion_r15247231
- [ ] Address https://github.com/sympy/sympy/pull/7608/files#discussion_r15974116
- [ ] https://github.com/sympy/sympy/pull/7608/files#discussion_r15974291
- [x] Use a single to_cnf and to_dnf and use dispatching to distinguish the two. 
It actually doesn't deviate from the standard. It is true that `expr.func` is the same as `type(expr)` for most classes, but the standard is that `expr.func(*expr.args) == expr`, which is what you are maintaining here. AppliedPredicate is another class that has a custom .func (and it's a class that you should be using in this module).