Infinities and NaNs are no longer rational

Infinities and NaNs are no longer rational

* libguile/numbers.c (scm_rational_p): Return #f for infinities and
  NaNs, per R6RS.  Previously it returned #t for real infinities
  and NaNs.  They are still considered real by scm_real `real?'
  however, per R6RS.  Also simplify the code.

  (scm_real_p): New implementation to reflect the fact that the
  rationals and reals are no longer the same set.  Previously it just
  called scm_rational_p.

  (scm_integer_p): Simplify the code.

* test-suite/tests/numbers.test: Add test cases for `rational?'
  and `real?' applied to infinities and NaNs.

* doc/ref/api-data.texi (Real and Rational Numbers): Update docs to
  reflect the fact that infinities and NaNs are no longer rational, and
  that `real?'  no longer implies `rational?'.  Improve discussion of
  infinities and NaNs.

* NEWS: Add NEWS entries, and combine with an earlier entry about
  infinities no longer being integers.