CRP / Dirichlet process mixture model

[jar398]

This is one of the last remaining examples that Metaprob needs to support in order to get up to Church or Venture level competence. We've been talking about it since the spring but there hasn't been an issue for it, so I'm adding it now. This task has been blocked for want of a mem implementation. Now that we have that, we can forge ahead.

Alexey's code, converted to clojure syntax, is here: https://github.com/probcomp/metaprob/blob/master/converted/examples/dpmem/example.clj

[zane]

Per @jar398, legacy code relating to this issue might be available in the python and converted directories at the legacy tag.