WL#5230: Replace strcasecmp() used for checking equality to a more efficient function

In /sql/*.cc we can find this style calls checking
equality throughout the code:

 if (!my_strcasecmp(s1->str, s2->str))

where s1 and s2 are of type LEX_STRING.

All of those calls can be faster if there is a check
for string lengths prior to doing the comparison:

  if (s1->len != s2->len ||
      !my_strcasecmp(s1->str, s2->str))

These two conditions should go into a separate inline function,
or into a macros, say lex_string_equal(), so the final calls
will look about like this:

  if (lex_string_equal(s1, s2))

Note, this optimization can be done only for "simple" collations
which only have "one character -> one weight" rules.
This optimization can NOT be done for "complex" collations
having expansions ("one to many") or contractions ("many to one").

As system_charset_info does not have these "complexities",
and most of the calls in /sql/*.cc is done for system_charset_info
values, this task can possibly give good overall performance improvement.


Also 'my_strncasecmp(...) == 0' notation is now used in other places,
making search for string comparisons harder. So another advantage of
this task is to make all comparison look in the same style.


References
==========
BUG#49501 Inefficient information_schema check (system collation)