Add read_element and read_element_ref methods to the IonReader trait, impls

[zslayton]

Currently the ability to read Element values is provided by implementations of the ElementReader trait. Historically, this included an implementation provided by ion-c-sys IonCElementReader as well as a native Rust implementation (NativeElementReader).

However, following PR #418, ion-rust no longer has a dependency on ion-c-sys. We should merge the functionality defined by ElementReader into IonReader, allowing applications to read the current value as either a known Rust type (read_f64()) or an Element (read_element()).

We should also offer methods to iterate across the elements of the current level.

Straw man method signatures

fn read_element(&mut self) -> IonResult<Element>;
fn read_element_ref<'a>(&'a mut self) -> IonResult<ElementRef<'a>>;
fn elements<'a>(&mut self) -> ElementIterator<'a>; // next() returns Option<IonResult<Element>> 
fn element_refs<'a>(&mut self) -> IonResult<ElementRefIterator<'a>>;

[zslayton]

Fixed in #485.