Improve readability of IonReaderContinuableCoreBinary.nextValue() and transcodeNextTo()

[tgregg]

These are lengthy methods with looping constructs and early returns. Figuring out a better factoring would improve readability and maintainability.

[tgregg]

Also apply this to IonReaderTextSystemX.transcodeNextTo().

Additionally, IonReaderContinuableCoreBinary.transcodeValueLiteral() and IonReaderTextSystemX.transcodeValueLiteral() are similar enough that we could consider making MacroAwareIonReader abstract and putting a generalized implementation in there.