Work with builtin functions.

 * Add file details/builtin_defs.h. File will contain macroes
   to work with builtin functions.
 * Move BUILTIN_EXPECT macro to file details/builtin_defs.h
 * Add macroes BUILTIN_LIKELY and BUILTIN_UNLIKELY.
   * BUILTIN_LIKELY is am alias for BUILTIN_EXPECT(val, !0).
   * BUILTIN_UNLIKELY is an alias for BUILTIN_EXPECT(val, 0).
 * Replace duplicate code with macro QUIT_ON_WRONG_WSUM.
Updated check_digit example.

 * Set checkdigit.h directory to sse2.
 * Fill val[10..16] with 1 (to be sure that there's no end of string).
 * Print error message on unknown identifier type.
Added UTF8 stuff.

 * Uploaded files string/utf8.{h,c}.
 * Add function which checks if a given string is a UTF8 string.
 * Add example.