refactor: fix logic for IBinaryInteger<T>.Factorial

The algorithm now starts at IBinaryInteger<T>.MultiplicativeIdentity rather than IBinaryInteger<T>.One

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- X1OD: `IBinaryInteger<T>.Factorial` now starts at `IBinaryInteger<T>.MultiplicativeIdentity` not
+`IBinaryInteger<T>.One`.
 - X10D: Removed `byte.Product`, `short.Product`, `ushort.Product`, `int.Product`, `uint.Product`, `long.Product`,
   and `ulong.Product`, in favour of `INumber<T>.Product`.
 - X10D: Removed `byte.RangeTo`, `short.RangeTo`, `ushort.RangeTo`, `int.RangeTo`, `uint.RangeTo`, `long.RangeTo`,

X10D/src/Math/BinaryIntegerExtensions.cs

@@ -69,13 +69,13 @@ public static class BinaryIntegerExtensions
             return 1;
         }
 
-        var result = 1L;
+        TInteger result = TInteger.MultiplicativeIdentity;
         for (TInteger i = TInteger.One; i <= value; i++)
         {
-            result *= long.CreateChecked(i);
+            result *= i;
         }
 
-        return result;
+        return long.CreateChecked(result);
     }
 
     /// <summary>