Simplify Not when applied to a Relational.

Replaces the Not object with a corresponding Relational object with equivalent meaning.

Note that I included the logic entirely within the Not class.  It may be preferable to instead to create something like an `_eval_Not` method for each relational object, and have Not call that method on any object where it's available.  But Relationals may be the only objects that need such special treatment, so the extra layer of indirection would be unneccessary and confusing.  In conclusion, feedback is appreciated!