Checking mb_*() via function_exits() should be cached

[miya0001]

Because function_exists() is expensive to run.

function mbfunctions_exist() {
    static $test = null;

    if ( $test !== null ) {
        return $test;
    }

    $test =  (
            function_exists( 'mb_convert_encoding' ) &&
            function_exists( 'mb_convert_kana' ) &&
            function_exists( 'mb_detect_encoding' ) &&
            function_exists( 'mb_strcut' ) &&
            function_exists( 'mb_strlen' )
    );

    return $test;
}

This ticket should be opened as soon as we add some other i18n fixes.

[miya0001]

This tickets should be opened after some tickets are merged.