MCOL-4633 Remove duplicate code for DECIMAL to int64_t rounding conversion

Detailed change list:

- Splitting out the narrow part of "class Decimal" into a separate class TDecimal64

- Adding a method TDecimal64::toSInt64Round()

- Reusing the method TDecimal64::toSInt64Round() in:
   * Func_cast_signed::getIntVal()
   * Func_char::getStrVal()
   * Func_elt::getStrVal()
   * makedate()
   * Func_maketime::getStrVal()

   Note, reusing this method in Func_char::getStrVal() also fixed this bug:
     MCOL-4634 CHAR(negativeWideDecimal) is not like InnoDB
   because the old code handled negative wide decimal values
   in a wrong way.

- Adding a new class TDecimal128 for symmetry.
  Moving a few wide decimal methods and constexpr's from Decimal to TDecimal128.
  The new class TDecimal128 does not do much at this point yet.
  Later we should be able to use TDecimal128 vs TDecimal64 in templates.

utils/funcexp/func_cast.cpp

@@ -183,26 +183,7 @@ int64_t Func_cast_signed::getIntVal(Row& row,
         case execplan::CalpontSystemCatalog::DECIMAL:
         case execplan::CalpontSystemCatalog::UDECIMAL:
         {
-            IDB_Decimal d = parm[0]->data()->getDecimalVal(row, isNull);
-
-            if (parm[0]->data()->resultType().colWidth == datatypes::MAXDECIMALWIDTH)
-            {
-                return static_cast<int64_t>(d.getPosNegRoundedIntegralPart(4));
-            }
-            else
-            {
-                double dscale = d.scale;
-                int64_t value = d.value / pow(10.0, dscale);
-                int lefto = (d.value - value * pow(10.0, dscale)) / pow(10.0, dscale - 1);
-
-                if ( value >= 0 && lefto > 4 )
-                    value++;
-
-                if ( value < 0 && lefto < -4 )
-                    value--;
-
-                return value;
-            }
+            return parm[0]->data()->getDecimalVal(row, isNull).toSInt64Round();
         }
         break;