8438: implement general cubic solution

The approach below cannot be used without special handling
to make sure that a valid pair of roots for _S and T are chosen
such that _S*T = -Q. [Thanks to Kalevi Suominen for noting why
the formulas below did not work as written for all cases].

** eqns (54)-(56) from http://mathworld.wolfram.com/CubicFormula.html

```python
f = Poly([1, -S(3)/2 - 7*I/2, -2, -3], x)
_, a2, a1, a0 = f.monic().all_coeffs()
coeff = I*sqrt(3)/2
Q = (3*a1 - a2**2)/9
R = (9*a2*a1 - 27*a0 - 2*a2**3)/54
D = Q**3 + R**2
_S = root(R + sqrt(D), 3)
T = root(R - sqrt(D), 3)
B = _S + T
A = _S - T
r = -a2/3 - B/2
bad = [-a2/3 + B, r + coeff*A, r - coeff*A]
```