Improved support for Unicode title case in Guile's string and character APIs.

Improved support for Unicode title case in Guile's string and character APIs.

* doc/ref/api-data.texi (Characters): Documentation for `char-titlecase'.
* doc/ref/api-i18n.texi (Character Case Mapping): Documentation for
  `char-locale-titlecase' and `string-locale-titlecase'.

* libguile/chars.c, libguile/chars.h (scm_char_titlecase, scm_c_titlecase): New
  functions.

* libguile/i18n.c, libguile/i18n.h (chr_to_case, scm_char_locale_titlecase,
  str_to_case, scm_string_locale_titlecase): New functions.
* libguile/i18n.c (scm_char_locale_downcase, scm_char_locale_upcase,
  scm_string_locale_downcase, scm_string_locale_upcase): Refactor to share code
  via chr_to_case and str_to_case, as appropriate.
* module/ice-9/i18n.scm (char-locale-title-case, string-locale-titlecase): New
  functions.

* libguile/srfi-13.c (string_titlecase_x): Use uc_totitle instead of uc_toupper.

* test-suite/tests/chars.test: Tests for `char-titlecase'.
* test-suite/tests/i18n.test: Tests for `char-locale-titlecase' and
  `string-locale-titlecase'.
* test-suite/tests/srfi-13.test: Tests for `string-titlecase'.