Separate `:=` tracking from values_as_in_model

[penelopeysm]

values_as_in_model is used to keep track of the model representation of random variables.

It is now also overloaded and used to keep track of things on the left-hand side of :=, i.e., deterministic random variables.

Now that values_as_in_model is being turned into an accumulator (#908) we should probably consider splitting this up, i.e. using separate accumulators for ~ and :=, or at the very least keeping some info about which one a given variable belonged to.

[yebai]

Related: #913