Adapt i18n string and char comparison funcs for Unicode

Adapt i18n string and char comparison funcs for Unicode

* libguile/i18n.c (RUN_IN_LOCALE_SECTION) [USE_GNU_LOCALE_API]: new
  macro for this define.  Should work the same as the !USE_GNU_LOCAL_API
  version
  (SCM_STRING_TO_U32_BUF, SCM_U32_BUF_FREE): helper macros
  (compare_u32_strings, compare_u32_strings_ci): new functions that
  compare scheme strings in the presence of the locale
  (u32_locale_casecoll): new helper function for compare_u32_strings_ci
  (compare_strings, compare_strings_ci): removed and replaced by
  compare_u32_strings and compare_u32_strings_ci
  (scm_string_locale_lt, scm_string_locale_gt, scm_string_locale_ci_lt)
  (scm_string_locale_ci_gt, scm_string_locale_ci_eq): modified to
  use new compare_u32_strings and compare_u32_strings_ci comparison
  functions
  (scm_char_locale_lt, scm_char_locale_gt, scm_char_locale_ci_lt)
  (scm_char_locale_ci_gt, scm_char_locale_ci_eq): modified to use
  new compare_u32_strings and compare_u32_strings_ci comparison
  functions