math.complex: give complex a present method

A present method for complex numbers:

    IN: C{ 3 3 } present .
    "3+3j"
    IN: { C{ 3 3 } C{ 0 0 } C{ 3.0 -5 } } "%[%s, %]\n" printf
    { 3+3j, 0, 3.0-5j }

I think it looks nice. :) And it's the format Python uses for complex numbers.