Improve standards conformance of string->number.

Improve standards conformance of string->number.

Fixes <http://bugs.gnu.org/11887>.

* libguile/numbers.c (mem2ureal): New argument 'allow_inf_or_nan'.
  Accept infinities and NaNs only if 'allow_inf_or_nan' is true and "#e"
  is not present.  Check for "inf.0" or "nan." case-insensitively.  Do
  not accept rationals with zero divisors.

  (mem2complex): Pass new 'allow_inf_or_nan' argument to 'mem2ureal',
  which is set if and only if a explicit sign was present.

* test-suite/tests/numbers.test ("string->number"): Add tests.