Optimize truncate, round, floor, and ceiling

Optimize truncate, round, floor, and ceiling

* libguile/numbers.c (scm_c_truncate): Use ceil (x) instead of
  -floor (-x).

  (scm_truncate_number): Implement directly instead of by checking the
  sign and using scm_floor or scm_ceiling.  Use scm_truncate_quotient
  for fractions.  Make extensible, so that new number types implemented
  in GOOPS will be able to do the job more efficiently, since it is
  often easier to implement truncate than floor or ceiling.

  (scm_round_number): Optimize fractions case by using
  scm_round_quotient.  Make extensible, so that new number types
  implemented in GOOPS will be able to do the job efficiently.

  (scm_floor, scm_ceiling): Optimize fractions case by using
  scm_floor_quotient and scm_ceiling_quotient, respectively.

* test-suite/tests/numbers.test: Add test cases.