Remove __rng__ and __sampler__ from INTERNALNAMES (#312)

Cf. [this discussion](#253 (comment)).  Also, derive deprecated names automatically.

In the next breaking release we could completely get rid of `DEPRECATED_INTERNALNAMES`.

Author: phipsgabler

src/compiler.jl

@@ -1,5 +1,5 @@
-const INTERNALNAMES = (:__model__, :__sampler__, :__context__, :__varinfo__, :__rng__)
-const DEPRECATED_INTERNALNAMES = (:_model, :_sampler, :_context, :_varinfo, :_rng)
+const INTERNALNAMES = (:__model__, :__context__, :__varinfo__)
+const DEPRECATED_INTERNALNAMES = (:_model, :_context, :_varinfo)
 
 """
     isassumption(expr)