6463: fix ordering of RealInterval.a and b
RootOf: raise ValueError if no index is obtained
RootOf: _eval_Eq is defined
8206: improve solving of relationals

- in solve_univariate_inequalities:
    - improved validation of ranges
    - improved handling of discontinuities
- in reduce_inequalities:
    - improved dispatching and error messages

- tests updated/added

One of the tests that formerly solve x < 3 in terms of re and im
parts was changed: those details will be apparent when someone
tries to substitute in a value for x: anything with an imaginary
part will fail. Therefore, it did not seem necessary to try enforce
that output.
rework solve_univariate
quick