Various optimizations and cleanups. "in" removed from GpuUInt128 to avoid defensive copies from IL.

PerfectNumbers.Core/HeuristicFailureReason.cs

@@ -0,0 +1,11 @@
+namespace PerfectNumbers.Core
+{
+    internal enum HeuristicFailureReason
+    {
+        PhiPartialFactorizationFailed,
+        StrictModeRequested,
+        HeuristicPipelineUnresolved,
+        HeuristicPipelineAssumedFullOrder,
+    }
+
+}