Internal bignum to int64_t convertion doesn't consider signness

[Takashi Kato]

The following should print minus second:

(date->time-utc (make-date 0 0 0 2 16 9 1945 3600))

But it prints plus second.

This is because Sg_BignumToS64 doesn't change signness when the element could is 1.

[Takashi Kato]

Exchanging sign when bignum count is 1 and minus sign (Fixes #138)

→ <<cset b65a775c8375>>