Improve handling of signed zeroes

Improve handling of signed zeroes

* libguile/numbers.c (scm_abs): (abs -0.0) now returns 0.0.  Previously
  it returned -0.0.  Also move the REALP case above the BIGP case,
  and consider it SCM_LIKELY to be REALP if not INUMP.
  (scm_difference): (- 0 0.0) now returns -0.0.  Previously it returned
  0.0.  Also make sure that (- 0 0.0+0.0i) will return -0.0-0.0i.

* test-suite/tests/numbers.test (abs, -): Add test cases, and change
  some tests to use `eqv?' instead of `=', in order to test exactness
  and distinguish signed zeroes.