Implement GPU pow2 heuristics with safe fallbacks

PerfectNumbers.Core/ULongExtensions.cs

@@ -56,6 +56,17 @@ public static class ULongExtensions
         return order;
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static int GetBitLength(this ulong value)
+    {
+        if (value == 0UL)
+        {
+            return 0;
+        }
+
+        return 64 - BitOperations.LeadingZeroCount(value);
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static ulong MultiplyShiftRight(ulong value, ulong multiplier, int shift)
     {