Some cases that Equality.simplify() could improve on

This is on top of #7960.  See final two commits.

Not very urgent, stuff like `Eq(x+1, y+1)` which user might like to simplify to `Eq(x, y)`.

The other example is take radicals like #7981 and put symbols inside.  `(lhs-rhs).equals(0)` (main tool from `_eval_sides()`) doesn't help here.