Use string accessors for string->number conversion

Use string accessors for string->number conversion

* libguile/numbers.c (scm_i_print_fraction): use string accessors
  (XDIGIT2UINT): use libunistring function
  (mem2uinteger, mem2integer, mem2decimal_from_point, mem2ureal)
  (mem2complex): take scheme string instead of c string; use accessors
  (scm_i_string_to_number): new function
  (scm_c_locale_string_to_number): use scm_i_string_to_number

* libguile/numbers.h: declaration for scm_i_string_to_number

* libguile/strings.c (scm_i_string_strcmp): new function

* libguile/strings.h: declaration for scm_i_string_strcmp