Replace Box<dyn Iterator> API for reading annotations

[zslayton]

Currently the IonReader trait includes an annotations() method that returns a Box<dyn Iterator<_>> over the current value's annotations:

ion-rust/src/stream_reader.rs

Lines 42 to 46 in 4f2eae6

	/// Returns an iterator that will yield each of the annotations for the current value in order.
	/// If there is no current value, returns an empty iterator.
	// TODO: When GATs are stabilized, we can change this to a known associated type that's generic
	// over the lifetime of &self.
	fn annotations<'a>(&'a self) -> Box<dyn Iterator<Item = IonResult<Self::Symbol>> + 'a>;

As the comment notes, we intended to replace this Box<dyn ...> with a generic associated type (GAT), so implementations of IonReader could supply a concrete Iterator implementation. However, we cannot do that for this particular method because we need the IonReader trait to be object safe. For the moment, traits that use GATs are not object safe. See also: rust-lang/rust#81823.

Iterating over a value's annotations shouldn't require the expense of Box<dyn>, so we need to replace this with a concrete implementation that would work for all reader implementations.

@almann had a good suggestion for this--add a method like this to IonReader:

trait IonReader {
    // ...
    fn read_annotation(&mut self) -> IonResult<Option<Self::Symbol>>;
}

This method would require reader implementations to maintain a small amount of state to track which annotations have been read. When reading the value foo::bar::5, calling read_annotation() once would return Ok(Some(foo)), calling it a second time would return Ok(Some(bar)), and calling it a third time would return Ok(None).

Having this method available in all IonReader implementations would allow us to provide a concrete, stack-allocated, statically-dispatched AnnotationsIterator type that worked for all readers.