Refactors the `structure` command to support recursive structures. These
are disabled for now, pending additional elaborator support in #5822.
This refactor is also a step toward `structure` appearing in `mutual`
blocks.
Error reporting is now more precise, and this fixes an issue where
general errors could appear on the last field. Adds "don't know how to
synthesize placeholder" errors for default values.
Closes#2512
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See RFC #3644 for a discussion of design choices.
Closes#3644
