[macro lang] Design principle around handling of annotations and nulls

[toddjonker]

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Now that you mention it, I feel icky about ignoring annotations. That seems likely to lead to users of the macro being surprised by data-loss.

We could use a written-down design principle on this sort of thing, as well as related topics like handling of nulls (typed, untyped, and mis-typed).

Originally posted by @toddjonker in https://github.com/amazon-ion/ion-docs/pull/249#discussion_r1222278807

At very least, the system macros should be consistent.

[toddjonker]

The keys provided to make_struct are an interesting and subtle case.