[Performance] XmlSerializer instantiation on every config load

[mfogliatto]

Description

XmlConfigurationLoader.ParseConfigFrom() creates a new XmlSerializer(typeof(ReferenceCopConfig)) on every call. XmlSerializer construction is expensive — it generates and compiles serialization assemblies at runtime.

Affected File

src/ReferenceCop/Configuration/XmlConfigurationLoader.cs — line in ParseConfigFrom() method.

Impact

• The Roslyn analyzer runs during compilation, potentially being invoked for every project in a solution.
• Each invocation pays the cost of XmlSerializer construction (reflection + dynamic code generation).
• In large solutions with many projects, this adds measurable overhead to build times.

Suggested Optimization

Cache the XmlSerializer instance in a static field:

private static readonly XmlSerializer ConfigSerializer = new XmlSerializer(typeof(ReferenceCopConfig));

private static ReferenceCopConfig ParseConfigFrom(Stream stream)
{
    return (ReferenceCopConfig)ConfigSerializer.Deserialize(stream);
}

XmlSerializer is thread-safe for deserialization, so a shared static instance is safe here.