Give users a way to implement IonInput for their own types

[zslayton]

Due to coherence limitations, the IonInput trait cannot provide blanket impls for all of the Read types that it could. This can lead to frustrating situations in which a user would like to read Ion from a common type but cannot without first wrapping it in a BufReader or similar.

This could be addressed by making IonStream and IonSlice pub (potentially without exposing any of their methods).