Remove unnecessary allocations in NUnitEqualityComparer

[manfred-brands]

The NUnitEqualityComparer on every instantiation creates a list if IChainComparer instances.
The only reason these are instances is because the constructor passes in the current instance of NUnitEqualityComparer
Refactor to static methods where this is passed as an extra argument to the Equal method instead.

This should reduce pressure on the garbage collector.