Return complex results when log is passed a negative number

[justinethier]

log is not defined on negative real numbers; if a negative value z is passed the result should be computed as though log received the complex number z+0i.

Consider:

cyclone> (log -1)
-nan.0
cyclone> (log -1+0i)
3.14159265358979i

See also: https://docs.scheme.org/surveys/complex-logarithm/