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