Avoid hand-implemented hashCode where possible

[tgregg]

I'm generally a little leery of hand-implementing hashCode repeatedly. I'd rather have utility methods that make this signficantly easier, e.g. a variadic function for convenience when hashing multiple fields and then an iterable function for hashing collections. Variadic calls iterable, and you should be able to do everything with those two.

Originally posted by @jobarr-amzn in https://github.com/amazon-ion/ion-java/pull/546#discussion_r1293935487