Reduce allocation threshold of BigIntegerCalculator

The aftermath of #2291 leads to the final conclusion that this is the
better value. A bigger threshold doesn't have a noticeable effect.

Author: axelheer

src/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.SquMul.cs

@@ -26,7 +26,7 @@ public unsafe static uint[] Square(uint[] value)
 
         // Mutable for unit testing...
         private static int SquareThreshold = 32;
-        private static int AllocationThreshold = 4096;
+        private static int AllocationThreshold = 256;
 
         [SecuritySafeCritical]
         private unsafe static void Square(uint* value, int valueLength,