<charconv>: Optimize _Integer_to_chars for bases 3,5,6,7,9 (#1622)

stl/inc/charconv

@@ -127,6 +127,17 @@ _NODISCARD to_chars_result _Integer_to_chars(
         } while (_Value != 0);
         break;
 
+    case 3:
+    case 5:
+    case 6:
+    case 7:
+    case 9:
+        do {
+            *--_RNext = static_cast<char>('0' + _Value % _Base);
+            _Value    = static_cast<_Unsigned>(_Value / _Base);
+        } while (_Value != 0);
+        break;
+
     default:
         do {
             *--_RNext = _Charconv_digits[_Value % _Base];