Optimization: make event arguments readonly structs

[SergeiPavlov]

The "Updated .NET Event pattern" (https://docs.microsoft.com/en-us/dotnet/csharp/modern-events) does not require event args to be inherited from EventArgs
It means they often (when they are sealed classes without hierarchy) can be converted into readonly structs to save GC work.

Also:

• Refactor event invocation to ?.Invoke( syntax