Support application implementation of IonReader and IonWriter

[almann]

We need to stabilize the interface so applications can implement it.

Imported from ION-183,IONJAVA-72

[jonwilsdon]

We need to investigate whether this is useful.

[pbcornell]

Here's an example of when this would be useful: amazon-ion/ion-hash-java#20

A schema-validating reader/writer would be another example.

[toddjonker]

Another use case: I have an object model whose data I want to expose via an IonReader, so the data can be consumed as if it were serialized, without going through the expense of doing so. This is effectively what can be done today for IonValues, but in user-space for other data models.