Define a type to represent any Ion Value

[R-maan]

I do not understand why we need a more formal type for this. ionItem is being used only for this test, should we create a public type?

Sorry, to clarify: I don't think we need a more formal type for this--it's fine as-is. However, I expect that at some point in the (distant?) future, ion-go will have a public-facing formal type that represents any Ion value. (Similar to ion-java's IonValue class.) If/when we add that, we should replace this so we're not maintaining two data types for the same thing.

Originally posted by @zslayton in #33