Fix bugs when negating SCM_MOST_POSITIVE_FIXNUM+1

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.