Support csharp7 Generalized async return type alternative to TaskInterfaceFactory.Create about itask

[jam40jeff]

Also, I will remove the CodeContracts stuff.

[binki]

I have verified that generalized return types allow one to write code like this:

[Test]
public async Task TaskInterfaceAsyncMethodBuilderTask()
{
    await TaskInterfaceAsyncMethodBuilderTaskMethodAsync().ConfigureAwait(false);
}

async ITask TaskInterfaceAsyncMethodBuilderTaskMethodAsync()
{
    await Task.Delay(50).ConfigureAwait(false);
}

[Test]
public async Task TaskInterfaceAsyncMethodBuilderResultTask()
{
    var value = await TaskInterfaceAsyncMethodBuilderResultTaskMethodAsync().ConfigureAwait(false);
    Assert.AreEqual(3, value);
}

async ITask<int> TaskInterfaceAsyncMethodBuilderResultTaskMethodAsync()
{
    var results = await Task.WhenAll(
        Enumerable.Range(0, 3).Select(async i =>
        {
            await Task.Delay(i);
            return i;
        }));
    return results.Length;
}

I’m about to submit a PR, but it is quite ugly because I don’t know how to use CodeContracts (and supposedly it doesn’t work with VS-2017/roslyn anyway because roslyn generated IL that is different from the old compilers, causing ccrewrite to fail to do things). Please review it and let me know if I can do things to make it work or if you can get the buildsystem to be compatible with VS-2017 and I can rebase my patches.

[jam40jeff]

Thanks for this! I have been meaning to look into this but I also was under the impression that interfaces were not supported. That's great to hear that they are!

I will update AppVeyor to use Visual Studio 2017 to build.

[binki]

Awesome! Let me know when you're done and I can try to fix my commits/PR. I noticed I messed up the newlines of the README and I probably need to fix the .cs files to follow project newline conventions and license headers, etc. I'll fix that all after you make your changes. Alternatively, feel free to just copy and paste the implementation over—I don't care that much about commits being in my name ;-).