Fixes and improvements to number-theoretic division operators

Fixes and improvements to number-theoretic division operators

* libguile/numbers.c (scm_euclidean_quotient, scm_euclidean_divide,
  scm_centered_quotient, scm_centered_divide): Fix bug in inum/inum
  case, where (quotient most-negative-fixnum -1) would not be converted
  to a bignum.

  (scm_euclidean_quotient): Be more anal-retentive about calling
  scm_remember_upto_here_1 after mpz_sgn, (even though mpz_sgn is
  documented as being implemented as a macro and certainly won't
  do any allocation).  It's better to be safe than sorry here.

  (scm_euclidean_quotient, scm_centered_quotient): In the bignum/inum
  case, check if the divisor is 1, since this will allow us to avoid
  allocating a new bignum.

  (scm_euclidean_divide, scm_centered_quotient, scm_centered_divide):
  When computing the intermediate truncated quotient (xx / yy) and
  remainder, use (xx % yy) instead of (xx - qq * yy), on the theory that
  the compiler is more likely to handle this case intelligently and
  maybe combine the operations.

  (scm_euclidean_divide): In the bignum/inum case, we know that the
  remainder will fit in an fixnum, so don't bother allocating a bignum
  for it.

  (scm_euclidean_quotient, scm_euclidean_remainder,
  scm_euclidean_divide, scm_centered_quotient, scm_centered_remainder,
  scm_centered_divide): Minor stylistic changes.

* test-suite/tests/numbers.test: Rework testing framework for
  number-theoretic division operators to be more efficient and
  comprehensive in its testing of code paths and problem cases.