`equal?' and `eqv?' are now equivalent for numbers

`equal?' and `eqv?' are now equivalent for numbers

Change `equal?' to work like `eqv?' for numbers.
Previously they worked differently in some cases, e.g.
when comparing signed zeroes or NaNs.  For example,
(equal? 0.0 -0.0) returned #t but (eqv? 0.0 -0.0)
returned #f, and (equal? +nan.0 +nan.0) returned #f
but (eqv? +nan.0 +nan.0) returned #t.

* libguile/numbers.c (scm_real_equalp, scm_bigequal,
  scm_complex_equalp, scm_i_fraction_equalp): Move to eq.c.

* libguile/eq.c (scm_real_equalp): Compare flonums using
  real_eqv instead of ==, so that NaNs are now considered
  equal, and to distinguish signed zeroes.

  (scm_complex_equalp): Compare real and imaginary
  components using real_eqv instead of ==, so that NaNs are
  now considered equal, and to distinguish signed zeroes.

  (scm_bigequal): Use scm_i_bigcmp instead of duplicating it.

  (real_eqv): Test for NaNs using isnan(x) instead of
  (x != x), and use SCM_UNLIKELY for optimization.

  (scm_eqv_p): Use scm_bigequal, scm_real_equalp,
  scm_complex_equalp, and scm_i_fraction_equalp to compare
  numbers, instead of inline code.  Those predicates now do
  what scm_eqv_p formerly did internally.  Replace if
  statements with switch statements, as is done in
  scm_equal_p.  Remove useless code to check equality of
  fractions with different SCM_CELL_TYPEs; this was for a
  tentative "lazy reduction bit" which was never developed.

  (scm_eqv_p, scm_equal_p): Remove useless code to check
  equality between inexact reals and non-real complex numbers
  with zero imaginary part.  Such numbers do not exist,
  because the current code is careful to never create them.

* test-suite/tests/numbers.test: Add test cases for
  `eqv?' and `equal?'.  Change existing test case for
  `(equal? +nan.0 +nan.0)' to expect #t instead of #f.

* NEWS: Add NEWS entries.