Support casing characters which map into multiple code points (bug#24603)

Implement unconditional special casing rules defined in Unicode standard.

Among other things, they deal with cases when a single code point is
replaced by multiple ones because single character does not exist (e.g.
‘fi’ ligature turning into ‘FL’) or is not commonly used (e.g. ß turning
into SS).

* admin/unidata/SpecialCasing.txt: New data file pulled from Unicode
standard distribution.
* admin/unidata/README: Mention SpecialCasing.txt.

* admin/unidata/unidata-get.el (unidata-gen-table-special-casing,
unidata-gen-table-special-casing--do-load): New functions generating
‘special-uppercase’, ‘special-lowercase’ and ‘special-titlecase’
character Unicode properties built from the SpecialCasing.txt Unicode
data file.

* src/casefiddle.c (struct casing_str_buf): New structure for
representing short strings used to handle one-to-many character
mappings.

(case_character_imlp): New function which can handle one-to-many
character mappings.
(case_character, case_single_character): Wrappers for the above
functions.  The former may map one character to multiple (or no)
code points while the latter does what the former used to do (i.e.
handles one-to-one mappings only).

(do_casify_natnum, do_casify_unibyte_string,
do_casify_unibyte_region): Use case_single_character.
(do_casify_multibyte_string, do_casify_multibyte_region): Support new
features of case_character.
* (do_casify_region): Updated to reflact do_casify_multibyte_string
changes.

(casify_word): Handle situation when one character-length of a word
can change affecting where end of the word is.

(upcase, capitalize, upcase-initials): Update documentation to mention
limitations when working on characters.

* test/src/casefiddle-tests.el (casefiddle-tests-char-properties):
Add test cases for the newly introduced character properties.
(casefiddle-tests-casing): Update test cases which are now passing.

* test/lisp/char-fold-tests.el (char-fold--ascii-upcase,
char-fold--ascii-downcase): New functions which behave like old ‘upcase’
and ‘downcase’.
(char-fold--test-match-exactly): Use the new functions.  This is needed
because otherwise fi and similar characters are turned into their multi-
-character representation.

* doc/lispref/strings.texi: Describe issue with casing characters versus
strings.
* doc/lispref/nonascii.texi: Describe the new character properties.

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