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
add FiniteDensity(dict)
skip SinglePSpace test_args
dict comprehension to dict(tuple generator)
Fix a test to test the right thing

See https://github.com/sympy/sympy/pull/1803#issuecomment-13903307.
Merge branch 'master' of github.com:sympy/sympy
Merge pull request #1811 from mrocklin/random-parameters

Bugfix in sympy.stats - add support for random parameters
3545: try unrad sooner

The test that was added used to hang while simplification occured;
The equation was f(q) + g(m) where f(q) was

-4*sqrt(m**2 + m**2/(4*q**2) - 2*m*q - m/(2*q) + q**2 + 1/4)

and g(m) was

sqrt(16*m**4 - 16*m**3 + 8*m**2 + 8*m*sqrt(4*m**4 - 4*m**2 + 8*m + 1) +
24*m + 4*sqrt(4*m**4 - 4*m**2 + 8*m + 1) + 4)

Trying to solve this for q gave solutions whose op count was 2920 and
unless check and simplify were off, seemed to hang the process.

A much better result is obtained (op count 16) by removing *all*
radicals, not only those containing the symbol of interest but takes
9 seconds.
3653: do not _invert polynomials or roots are lost