Change Unicode collation implementation from "handler" to "inline" style

axel found a performance bottleneck in the Unicode collation implementation (MDEV-16413).

Under terms of this task, we'll reorganize the code implementing Unicode collations (such as utf8_unicode_ci) to a new style which will change virtual function calls to inline-ing:

The old style

• There is one copy of every collation function (strnncoll(), strnncollsp(), hash_sort(), strnnxfrm()), e.g.:

  static int my_strnncollsp_uca(CHARSET_INFO *cs,

  my_uca_scanner_handler *scanner_handler,

  const uchar *s, size_t slen,

  const uchar *t, size_t tlen)

• Character set-specific routines are passed in scanner_handler
• scanner_handler->next() is called virtually (i.e. via a pointer to a function)
• scanner_handler->next() itself calls cs->cset->mb_wc() virtually

The new style

• There are multiple implementations of the functions, one function per character set.
• There is a shared file ctype-uca.ic, which is included multiple times, one time per each character set.
• Character set specific information is passed in macros:

  #include "ctype-utf8.h"

  #define MY_FUNCTION_NAME(x) my_uca_ ## x ## _utf8mb3

  #define MY_MB_WC(scanner, wc, beg, end) (my_mb_wc_utf8mb3_quick(wc, beg, end))

  #define MY_LIKE_RANGE my_like_range_mb

  #include "ctype-uca.ic"

• There are inline my_mb_wc_CSNAME_quick() implementations in new header files: ctype-utf8.h, ctype-ucs2.h, ctype-utf16.h, ctype-utf32.h

The old version generated smaller amount of executable code, but was slower.
The new version will generate more code, but will be much faster: there will be no any virtual function calls. All calls inside new functions will be done either using inline or at least statically.

Part#2: additional changes:

• Add fast paths to handle ASCII characters
• Add dedicated MY_COLLATION_HANDLERs for collations with no contractions (for utf8 and for utf8mb4 character sets). The choice between the full-featured handler and the "no contraction" handler should be made at the collation initialization time.