Fixed map being stuck in an infinite loop on occasion.
Added _list/tabulate.

This is going to be similar to _list/tabulate from the toolbox, except that it
produces JSON-y tables. I feel more comfortable hoisting this anew instead of
just importing the toolbox one.
Regex, SQL-like expressions and linked documents have yet to be added.
Moved 'documents' into top-level filter.

..and fixed list helper function to work with object keys.