polarify doesn't descend beneath exprs
support rvs in rvs
check pspace always receives symbol

This is a common fail-point for poorly behaved SymPy Expr code.
density evaluate=False
RandomSymbol._diff_wrt
solvers._ispow checks isinstance Expr
Density(constant) -> Lambda(DiracDelta)
RandomSymbol.is_Atom
given simple case of Eq(Rv, expr) -> subs
ProductContinuousPSpace subs in all symbols in pdf
conjugate prior test
fix doctest in Poisson