(- 0.0 0+i) should return 0.0-1.0i

[Takashi Kato]

The above expression returns 0.0+1.0i. This is wrong.

[Takashi Kato]

(- 1/3 0+i) also returns incorrect sign on imaginary part.

[Takashi Kato]

Negating imaginary part of complex number during subtraction (Fixes #113)

→ <<cset 8ef1f865137f>>