Conversions with multiple inputs and outputs

Currently, conversion functions only take one input / output. Will be critical to allow multiple. Impl is easy, api less so (lots to consider: how the conversions, the js transformer, and the cli should handle all this). Will use this issue to propose a few api changes and then select one.

Start by reviewing the existing conversion API, the JavaScript transformer, and the CLI to understand how single inputs and outputs are currently handled. The work is done when a multiple-input and multiple-output API is selected and its behavior is defined consistently across those areas.

Written by the indexing model from the issue text.