remove auto-switch between char and string based on the string length

1. it is against sql way, auto-detection should use metadata, not data,
consider:

create table t1 (qwe varchar(10)) as values ('qwe'),('qw'),('q'),('werty');
select sformat('{:*>5s}', qwe) from t1;

this will auto-break on the third row.

2. using max_char_length() instead of length() fixes that, but
there's a second big issue, fmt < 8.0 doesn't natively support unicode,
so {:c} would only work for one-byte strings, for 'a', not for 'я'

because of all that let's always format strings as strings.
{:c} will only now work for numbers and still only in the ascii range.

mysql-test/main/func_sformat.result

@@ -222,8 +222,8 @@ Number: **4**
 select sformat('{:02} - {:02} - {:02}', 1, 2, 3);
 sformat('{:02} - {:02} - {:02}', 1, 2, 3)
 01 - 02 - 03
-select sformat('Character {:c}', 'h');
-sformat('Character {:c}', 'h')
+select sformat('Character {:c}', 104);
+sformat('Character {:c}', 104)
 Character h
 select sformat('String {:s}', 'hello');
 sformat('String {:s}', 'hello')
@@ -391,17 +391,17 @@ Warning	4183	SFORMAT error: format specifier requires numeric argument
 # Table Format Test Cases
 #
 create table t1 (pformat text, data1 FLOAT, data2 INT,
-data3 VARCHAR(30), data4 CHAR);
-insert into t1 values ('{:.2f} {: } |{:*^15}| {:c}',  1.11,  1, 'example I',   'A'),
-('{:.1f} {:-} |{:^15}| {:c}',   -2.22, -2, 'example II',  'B'),
-('{:.2f} {:+} |{:<15}| {:c}',    3.33,  3, 'example III', 'C'),
-('{:.4f} {:d} |{:^15}| {:c}',   -4.44, -4, 'example IV',  'D');
-select sformat(pformat, data1, data2, data3, data4) from t1;
-sformat(pformat, data1, data2, data3, data4)
-1.11  1 |***example I***| A
--2.2 -2 |  example II   | B
-3.33 +3 |example III    | C
--4.4400 -4 |  example IV   | D
+data3 VARCHAR(30));
+insert into t1 values ('{:.2f} {: } |{:*^15}',  1.11,  1, 'example I'),
+('{:.1f} {:-} |{:^15}',   -2.22, -2, 'example II'),
+('{:.2f} {:+} |{:<15}',    3.33,  3, 'example III'),
+('{:.4f} {:d} |{:^15}',   -4.44, -4, 'example IV');
+select sformat(pformat, data1, data2, data3) from t1;
+sformat(pformat, data1, data2, data3)
+1.11  1 |***example I***
+-2.2 -2 |  example II   
+3.33 +3 |example III    
+-4.4400 -4 |  example IV   
 drop table t1;
 create table t2 (a double);
 insert into t2 values (3.14159265358979323846);

mysql-test/main/func_sformat.test

@@ -118,7 +118,7 @@ select sformat('Num {:L}', 13800000000);
 select sformat('Num [{:20}]', 42);
 select sformat('Number: {:*^{}}', 4, 5);
 select sformat('{:02} - {:02} - {:02}', 1, 2, 3);
-select sformat('Character {:c}', 'h');
+select sformat('Character {:c}', 104);
 select sformat('String {:s}', 'hello');
 select sformat('Large {0:+010.4g}', 392.64);
 select sformat('Large {:g}', 392.65);
@@ -171,12 +171,12 @@ echo #;
 echo # Table Format Test Cases;
 echo #;
 create table t1 (pformat text, data1 FLOAT, data2 INT,
-                 data3 VARCHAR(30), data4 CHAR);
-insert into t1 values ('{:.2f} {: } |{:*^15}| {:c}',  1.11,  1, 'example I',   'A'),
-                      ('{:.1f} {:-} |{:^15}| {:c}',   -2.22, -2, 'example II',  'B'),
-                      ('{:.2f} {:+} |{:<15}| {:c}',    3.33,  3, 'example III', 'C'),
-                      ('{:.4f} {:d} |{:^15}| {:c}',   -4.44, -4, 'example IV',  'D');
-select sformat(pformat, data1, data2, data3, data4) from t1;
+                 data3 VARCHAR(30));
+insert into t1 values ('{:.2f} {: } |{:*^15}',  1.11,  1, 'example I'),
+                      ('{:.1f} {:-} |{:^15}',   -2.22, -2, 'example II'),
+                      ('{:.2f} {:+} |{:<15}',    3.33,  3, 'example III'),
+                      ('{:.4f} {:d} |{:^15}',   -4.44, -4, 'example IV');
+select sformat(pformat, data1, data2, data3) from t1;
 drop table t1;
 
 create table t2 (a double);

sql/item_strfunc.cc

@@ -1401,10 +1401,7 @@ String *Item_func_sformat::val_str(String *res)
         delete [] vargs;
         return NULL;
       }
-      if (parg->length() == 1)
-        vargs[carg-1]= fmt::detail::make_arg<ctx>(parg->ptr()[0]);
-      else
-        vargs[carg-1]= fmt::detail::make_arg<ctx>(*parg);
+      vargs[carg-1]= fmt::detail::make_arg<ctx>(*parg);
       break;
     case TIME_RESULT: // TODO
     case ROW_RESULT: // TODO