Convert Mimir <-> Vizual interface to RA

[okennedy]

At present, Vizier converts Vizual scripts into SQL before passing it to Mimir.

https://github.com/VizierDB/web-api/blob/master/vizier/workflow/vizual/mimir.py

SQL is a recipe for parsing errors, unescaped strings, and other forms of suffering. Fortunately, Mimir supports a more direct interface: Relational Algebra encoded via JSON.