Implement Measurable Stacks

Implement _get_measurable_outputs for DiscreteMarkovChainFactory

Return updates created by discrete_markov_chain

Add HMM-constructing helper functions and Ops

Update imports for relocated C types

Do not replace valued RVs in `naive_bcast_rv_lift`

Take the abs value of the Jacobian determinant in the default transform

Add a Dirac-delta distribution

Remove unnecessary assert in naive_bcast_rv_lift

Fix scalar mixture distributions with non-scalar index case