Removes multimixture, the long way

[joshuathayer]

This is funny PR. It's the history of adding CGPM, multimix, and search-by-example to the Metaprob examples directory, refactoring search-by-example, adding profiling code, doing various optimizations, then moving all of it to a different repo.

The end result is basically just removing the multimixture example from this project, plus some minor performance changes.