IonHashWriterBuilder shouldn't require a delegate IonWriter

[toddjonker]

I have an alternate representation of some Ion data. Consider it a POJO for simplicity. I want to compute an Ion hash for the data, but I don't want to generate any serialized form of the data. I just want the digest at the end.

Today I'm forced to serialize the data into either text or binary, and then ignore the output, but that's a waste of resources. If there were a null-writer available, that might be useful. I cannot create my own, however, because it's a violation of the IonWriter contract to do so. (I'm going to ignore the fact that this library also violates that contract; that's a separate issue.)

Really I just want to not call IonHashWriterBuilder.withBuilder() and do everything else as normal.