Relational: respect simplify parameters (ratio, measure), refactor

Make sure the ratio and measure parameters are forwarded to internal
simplify calls.  Rename `_eval_sides` to `_eval_simplify_helper`.
Equality: XFAIL test that should simplify() to true but doesn't

Putting `x` inside some expressions that otherwise use the
`.equals(0)` code to simplify means they don't simplify to True.