generate IParams for users in smart way

[adamsitnik]

This change generates the right IParam for the users for any complex types returned by [ArgumentsSource] and [ParamsSource]

I was tired of writing another IParam when I realized that I don't need to do that.

Idea:

1. generate code which is going to call the [Source] getter/method
2. it always returns IEnumerable so we can call ToArray() on it
3. use array indexer to get the argument

Examples and auto-generated code:

[ParamsSource(nameof(DictionaryAsParam))]
public Dictionary<int, string> DictionaryParamInstance;

public IEnumerable<object> DictionaryAsParam => new object[] { new Dictionary<int, string>() { { 1234, "it's an instance getter" } } };

generates:
Runnable_0 instance = new Runnable_0(); 
instance.DictionaryParamInstance= (System.Collections.Generic.Dictionary<System.Int32, System.String>)instance.DictionaryAsParam().ToArray()[0];;

[Benchmark]
[ArgumentsSource(nameof(NonPrimitive))]
public void Simple(SomeClass someClass, SomeStruct someStruct)

public IEnumerable<object[]> NonPrimitive()
{
    yield return new object[] { new SomeClass(Enumerable.Range(0, 100).ToArray()), new SomeStruct(100) };
    yield return new object[] { new SomeClass(Enumerable.Range(0, 1000).ToArray()), new SomeStruct(1000) };
}

__argField0 = (BenchmarkDotNet.IntegrationTests.WithNonPrimitiveArgumentsSource.SomeClass)NonPrimitive().ToArray()[0][0];;
__argField1 = (BenchmarkDotNet.IntegrationTests.WithNonPrimitiveArgumentsSource.SomeStruct)NonPrimitive().ToArray()[0][1];;

[adamsitnik]

@AndreyAkinshin please take a look (it's 3:30 am here and I am falling asleep and I need really good review)

[adamsitnik]

It's blocking me, I am merging it.

[AndreyAkinshin]

LGTM