Any complex number can be (un)archived properly.

Any complex number can be (un)archived properly.

A complex number can have an exact real part and inexact (floating point)
imaginary part, and vice a versa. Handle these cases properly in the archiving
code instead of bailing out with a bizzare error message.

Thanks to Chris Bouchard for reporting the bug.

NOTE: this fix changes the format of GiNaC archives, so formally it breaks
the binary compatibility. However, "mixed" complex numbers (i.e.  ones with
exact real part and inexact imaginary part) can not be archived with previous
versions of GiNaC, so there's nothing to break.