Improve type safety for custom graph stages

[marcpiechura]

In order to make the API consistent I've used Inlet<T> and Outlet<T> on all public methods, even if they doesn't really need it, e.g. IsAvailable .
For those methods which are used internally and where no type argument could be provided, I've created a private version without type argument, e.g. Pull.

Since this is quite a breaking change we need to decide if we wait for the next major version or release it with the next minor since it would prevent some ugly runtime exceptions.

closes #3231

[Horusiath]

Are you sure, this is right? It looks like less generic version.

[marcpiechura]

Yup, the previous more generic version was actually a „bug“, only prevented from happening due to the fact that the public api already restricts it to two different types.
I got a compilation error at the following line
„Push(killSwitch.Out2, Grab(killSwitch.In2)),“
which makes it obvious that both, the inlets and outlets, have to be of the same type.

[marcpiechura]

The type names are a bit confusing, T1 instead of TIn and T2 instead of TOut would make more sense, so I’ll update them when if fix the other issue with the performance test project.

[marcpiechura]

This is the public api I’m referring to in the other comment

[Aaronontheweb]

@marcpiechura looks like we have some merge conflicts with the API approval

[marcpiechura]

fixed the conflicts

[Aaronontheweb]

@marcpiechura

NBench.NBenchException: Error occurred during $Akka.Streams.Tests.Performance.InterpreterBenchmark+Graph_interpreter_100k_elements_with_1_identity RUN. ---> System.Reflection.TargetException: Non-static field requires a target.
   at System.Reflection.RtFieldInfo.CheckConsistency(Object target)
   at System.Reflection.RtFieldInfo.InternalSetValue(Object obj, Object value, BindingFlags invokeAttr, Binder binder, CultureInfo culture, StackCrawlMark& stackMark)
   at System.Reflection.RtFieldInfo.SetValue(Object obj, Object value, BindingFlags invokeAttr, Binder binder, CultureInfo culture)
   at Akka.Streams.Tests.Performance.InterpreterBenchmark.GraphDataSource`1..ctor(String toString, T[] data)
   at Akka.Streams.Tests.Performance.InterpreterBenchmark.<>c__DisplayClass4_0.<Execute>b__0(TestSetup setup, Func`1 lastEvents)
   at NBench.Sdk.Benchmark.WarmUp()
   --- End of inner exception stack trace ---

[Aaronontheweb]

Error was from Performance.InterpreterBenchmark+Graph_interpreter_100k_elements_with_1_identity