MDEV-26664 Store UUIDs in a more efficient manner

UUID values

  llllllll-mmmm-Vhhh-vsss-nnnnnnnnnnnn

are now stored as

  nnnnnnnnnnnn-vsss-Vhhh-mmmm-llllllll

inside the record:

- the groups (segments separated by dash) are reordered right-to-left.
- the bytes inside the groups are not reordered (stored as before,
  in big-endian format).

This provides a better sorting order: the earlier UUID was generated,
the higher it appears in the ORDER BY output.

Also, this change enables a good key prefix compression,
because the constant part is now in the beginning, while
the non-constant part (the timestamp) is in the end.

sql/sql_string.h

@@ -484,6 +484,11 @@ public:
     if (str.Alloced_length)
       Alloced_length= (uint32) (str.Alloced_length - offset);
   }
+  LEX_CSTRING to_lex_cstring() const
+  {
+    LEX_CSTRING tmp= {Ptr, str_length};
+    return tmp;
+  }
   inline LEX_CSTRING *get_value(LEX_CSTRING *res)
   {
     res->str=    Ptr;