Split out from SERVER-79367 since they could be more generally useful. See that ticket for a summary of the original motivation.

Current approach is to do rudimentary type detection, and apply simplifications when we know the output types:

• isObject(<known type>) -> true/false (can do similar for isArray/exists)
• setField(<const object/var -> const object>, "field", <value>) -> newObj(<fields from const obj>, "field", value)
• setField(newObj(...), "field", <value>) -> newObj(..., "field", <value>)