Fix bugs when negating SCM_MOST_POSITIVE_FIXNUM+1
* libguile/numbers.c (scm_difference, scm_product):
Fix bugs when negating SCM_MOST_POSITIVE_FIXNUM+1,
aka -SCM_MOST_NEGATIVE_FIXNUM. Previously, these cases
failed to normalize the result to a fixnum, causing
`=', `eqv?' and `equal?' to fail, e.g.:
(= most-negative-fixnum (- 0 (- most-negative-fixnum)))
(= most-negative-fixnum (* -1 (- most-negative-fixnum)))
(= most-negative-fixnum (* (- most-negative-fixnum) -1))
* test-suite/test/numbers.test: Add test cases to detect
bugs when negating SCM_MOST_POSITIVE_FIXNUM+1 and
SCM_MOST_NEGATIVE_FIXNUM by various methods.