This PR adds a performance benchmarking and some performance gains and reduction in memory dealing with formatted cells (and cell cloning).

Performance Benchmarking and Testing

• Added benchmark.net test project. With benchmark to validate performance change. Is marked excluded to avoid running with standard test suite.

Code refactoring.

Focused around some hotpaths dealing with formatted cells. As we use the Sister project ClosedXML.Reports and experience performance issues there. (Note: A lot of time for me is seen in FormatCell Set OutsideBorder , maybe there are some future savings there.

The changes primarily aim to reduce memory allocation and tweak performance.
In broad strokes they are:

• reduce use of ToArray/ToList when not required to avoid allocation and GC perf.
• change XLRepositoryBase to take in a WeakReference<Value> and reduce calls via TryGet, GetEnumerator also now yields.
• CollectChildrenRecursively(XLStylizedBase parent) should be faster than the HashSet, reduces recursion and uses the Stack

Other

minor style changes - feel free to ignore. (Prefer modern language features, use keyword over type name) etc.

Testing results

Before changes

After changes