Fix numerator and denominator handling of signed zeroes and infinities.

Fix numerator and denominator handling of signed zeroes and infinities.

* libguile/numbers.c (scm_numerator, scm_denominator): Handle signed
  zeroes and infinities in accordance with the corresponding R6RS flonum
  procedures.

* module/rnrs/arithmetic/flonums.scm (flnumerator, fldenominator):
  Remove special handling of infinities.

* test-suite/tests/numbers.test (numerator, denominator): Add tests.
  Convert existing tests to use 'pass-if-equal'.

* test-suite/tests/r6rs-arithmetic-flonums.test (flnumerator): Fix
  broken test of (flnumerator -0.0).