Implement characteristic polynomial

Implement characteristic polynomial using `berkowitz`'s algorithm. Only the coefficients of the characteristic polynomial are returned, not the characteristic polynomial itself.
Okay, I'll document it. `B` has a list of coefficients, in the decreasing order of the power of monomials. For example, if `B = transpose([1, -2, 3])` then the relevant polynomial is `x^2 - 2x + 3`. We don't need to multiply the elements of `B`.