Improved modelling syntax

[Joshuaalbert]

• We want to use jaxctx, which is the JAXNS modelling framework isolated from jaxns.
• Avoid prior model retracing #207 will be resolved.
• Explicitly pass into model args, to avoid closure problems.

[tavin]

I don't suppose there is a world in which numpyro and jaxns converge somehow on the same paradigm?

After playing with numpyro's jaxns wrapper I gave up on it and rewrote my code for jaxns. I can't say which I prefer. They're both elegant in their own way.

[Joshuaalbert]

I think it's not likely they will unify. However, it might be possible to "convert" a jaxns model into numpyro model automatically.