Pools made thread static. Various small optimizations. "in" parameter added to read-only method parameters

PerfectNumbers.Core/FactorEntry128.cs

@@ -0,0 +1,8 @@
+namespace PerfectNumbers.Core;
+
+public readonly struct FactorEntry128(in UInt128 value, int exponent)
+{
+	public readonly UInt128 Value = value;
+
+	public readonly int Exponent = exponent;
+}