Detection logic for complex-valued math sequences

[Technologicat]

Currently, s() can handle only real-valued inputs. Complex-valued sequences can be created by composing s(re) + 1j*s(im) or s(mag) * math.exp(1)**(1j*s(arg)), but this is inconvenient.

So, it would be nice to have support for complex-valued sequences.

The complication is that logarithms behave less nicely for complex inputs. If we want to keep supporting both numeric (float, mpmath) and symbolic (SymPy) input, different logic may be needed to correctly detect the sequence for these cases.

Also, just for a pie in the sky, automatic OEIS lookup (if an internet connection is available) would be nice for integer inputs. (Maybe as a separate feature; for many sequences, only the first few elements are known anyway.)