MDEV-20017 Implement TO_CHAR() Oracle compatible function

TO_CHAR(expr, fmt)
- expr: required parameter, data/time/timestamp type expression
- fmt: optional parameter, format string, supports
  YYYY/YYY/YY/RRRR/RR/MM/MON/MONTH/MI/DD/DY/HH/HH12/HH24/SS and special
  characters. The default value is "YYYY-MM-DD HH24:MI:SS"

In Oracle, TO_CHAR() can also be used to convert numbers to strings, but
this is not supported. This will gave an error in this patch.

Other things:
- If format strings is a constant, it's evaluated only once and if there
  is any errors in it, they are given at once and the statement will abort.

Original author: woqutech
Lots of optimizations and cleanups done as part of review

sql/sql_string.cc

@@ -502,16 +502,16 @@ bool String::set_ascii(const char *str, size_t arg_length)
 
 /* This is used by mysql.cc */
 
-bool Binary_string::fill(uint32 max_length,char fill_char)
+bool Binary_string::fill(size_t max_length,char fill_char)
 {
   if (str_length > max_length)
-    Ptr[str_length=max_length]=0;
+    Ptr[str_length= (uint32) max_length]=0;
   else
   {
     if (realloc(max_length))
       return TRUE;
     bfill(Ptr+str_length,max_length-str_length,fill_char);
-    str_length=max_length;
+    str_length= (uint32) max_length;
   }
   return FALSE;
 }