use less python

[odanoburu]

change mill.py to just read the JSON output by mill and produce RDF. (boostrap_legacy_rdf will have to create the same JSON from the RDF for bootstrapping, if we care about this)

we can then parse the JSON with haskell and serialize it to text, with much better performance, and better libraries for testing that nothing breaks. (and then mill is much more self-contained)