SMC meta-program for the HMM example

[jar600]

VKM: "defining an SMC meta-program for the HMM example from Marco" https://github.com/probcomp/metaprob-clojure/milestones/5

SMC = sequential Monte Carlo

[zane]

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