Bug fix: Several bugs were fixed involving signed zero and branch cuts
in complex numbers:

- -0.+i is now correctly parses as negative zero real part; previously
   the negative was discarded and it was parsed as positive zero.
- -0.i is now correctly printed as such, not as `+-0.i'.
- (atan -2i) now returns the same result as (atan 0.-2i), not (atan -0.-2i).
