Clarify edge cases when `parse_ion` encounters something that looks like a system value

[popematt]

The parse_ion macro must always produce user values, but there are some edge cases to work out.

• parse_ion doesn't (and shouldn't) know at what depth it was invoked, so anything in its output that looks like a system value must be annotated with $ion_literal
• practically speaking, it might be easier to say that the parse_ion reader does not perform the step of removing $ion_literal from the expanded values so anything that is annotated with $ion_literal in the embedded document will also be annotated with $ion_literal in the expansion. However, this is subtly different because it means that $ion_literal::1 will be passed through the expansion as-is, whereas if we say that the parse_ion expansion annotates anything that looks like a system value, then it would be read as 1 and put into the expansion output as 1.
• This may lead to unexpected behavior, depending on the resolution of Document $ion_literal and work out edge cases. #322. If parse_ion is invoked in a container, will the user expect to see $ion_literal annotations in the result?

The questions that need to be answered are:

• Can we define parse_ion such that it just passes through all $ion_literal annotations? (Instead of having to determine whether a value looks like a system value)?
• Can Document $ion_literal and work out edge cases. #322 be resolved in a way such that $ion_literal to be treated the same at any depth—not just at the top level?