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.h

@@ -740,7 +740,7 @@ public:
     thread_specific= s.thread_specific;
     s.alloced= 0;
   }
-  bool fill(uint32 max_length,char fill);
+  bool fill(size_t max_length,char fill);
   /*
     Replace substring with string
     If wrong parameter or not enough memory, do nothing