fix string->number for bases > 16

fix string->number for bases > 16

* libguile/numbers.c (scm_iuint2str): Add an assertion on the domain of
  the radix. Use the number_chars table to write the string, instead of
  doing strange math. Same effect, though.
  (mem2uinteger, char_decimal_value): Change logic to allow all ascii
  alphabetic chars as decimals, not just a-f. Thanks to Nils Gey for the
  report.

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