@tarleb A couple small notes about the Lua interface and documentation:

1. I always have lots of trouble, when I'm looking up documentation, distinguishing the Image function that is used as a handler (https://pandoc.org/lua-filters.html#type-image) and the Image function that is used to produce an image (https://pandoc.org/lua-filters.html#pandoc.image). How can we distinguish these to avoid confusion? Would it help to make the title heading pandoc.Image(..) instead of Image(...)? Should we have two sections, Handlers and Builders?

2. The docs for the Header handler mistakenly say it creates an element. https://pandoc.org/lua-filters.html#type-header

3. For DefinitionList (both handler and builder), there is no documentation about what type the contents list elements have.

4. I wonder whether we need all the namespaces? e.g. walk_block is in the main pandoc namespace, while blocks_to_inlines is in pandoc.util; read is in pandoc, while citeproc is in pandoc.utils -- this is hard to remember, and is there a good reason for it? Also maybe Version could go in pandoc? The existing modules could be preserved and the names reexported from pandoc, for backwards compatibility. The mediabag, path, etc. modules do seem necessary.