Implement `finite?' in core and fix R6RS `finite?' and `infinite?'

Implement `finite?' in core and fix R6RS `finite?' and `infinite?'

* libguile/numbers.c (scm_finite_p): Add new predicate `finite?' from
  R6RS to guile core, which returns #t if and only if its argument is
  neither infinite nor a NaN.  Note that this is not the same as (not
  (inf? x)) or (not (infinite? x)), since NaNs are neither finite nor
  infinite.

* test-suite/tests/numbers.test: Add test cases for `finite?'.

* module/rnrs/base.scm: Import `inf?' as `infinite?' instead of
  reimplementing it.  Previously, the R6RS implementation of
  `infinite?' did not detect non-real complex infinities, nor did it
  throw exceptions for non-numbers.  (Note that NaNs _are_ considered
  numbers by scheme, despite their name).

  Import `finite?' instead of reimplementing it.  Previously, the R6RS
  implementation of `finite?' returned #t for both NaNs and non-real
  complex infinities, in violation of R6RS.

* NEWS: Add NEWS entries, and reorganize existing numerics-related
  entries together under one subheading.

* doc/ref/api-data.texi (Real and Rational Numbers): Add docs for
  `finite?' and scm_finite_p.