Reimplement idbl2str number printer.

Reimplement idbl2str number printer.

Fixes <http://bugs.gnu.org/13757>.

* libguile/numbers.c (idbl2str): Reimplement.
  (mem2decimal_from_point): Accept negative exponents larger than
  SCM_MAXEXP that produce subnormals.
  (SCM_MAX_DBL_PREC): Removed preprocessor macro.
  (scm_dblprec, fx_per_radix): Removed static variables.
  (init_dblprec, init_fx_radix): Removed static functions.
  (scm_init_numbers): Remove initialization code for 'scm_dblprec'
  and 'fx_per_radix'.

* test-suite/tests/numbers.test ("number->string"): Restore tests that
  previously failed.  Remove comments about problems in the number
  printer that are now fixed.