Give a warning when the [Benchmark] method is static

[adamsitnik]

BDN supports only instance methods, when our users try to do sth like:

public class Some
{
   [Benchmark]
   public static void This() { }
}

they get a silent error.

@AndreyAkinshin or maybe we should allow for static method calls?

[AndreyAkinshin]

@adamsitnik, I'm not sure that static benchmarks is a good idea. It will complicate our generation template and it can introduce some undesirable effects in nanobenchmarks (such a feature should be carefully checked). Let's just warn users about an incorrect signature.

[Rizzen]

I can do it.

[AndreyAkinshin]

@Rizzen thanks!