Remove dependency on `IonValue`/`IonSystem` from `SqlParser`, `SqlLexer` and `Token`

[dlurton]

Since we're generally moving toward IonElement over IonValue in many places of PartiQL, it would be good if we can eliminate the dependency on IonSystem from the SqlParser and SqlLexer classes. We should also change Token.value to be of type AnyElement as well.

I include these three units of work under the same ticket since I think it will be difficult to accomplish each individually.