returns the vector of (complex) roots of the (complex)
polynomial: Sum(a(i)*x^(i-1)) using Laguerre's method.
The roots are sorted in order of increasing real parts. Call with
non-zero imsort keyword to sort into increasing imaginary parts.
See Numerical Recipes (Press, et. al., Cambridge Univ. Press 1988),
interpreted function, defined at i/zroots.i line 10