Relational: simplify some expressions with variables on both sides

Simplify the difference in the `_eval_sides` code.  This can cancel
`x` from both sides for example.  Add some tests that fail to simplify
without this.
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 use the `.equals(0)` code
means they don't simplify to True.