Synchronous postcondition for async generic method

[SergeyTeplyakov]

ccrewrite generates special 'async closure', moves postcondition for task-based method there (both with async keyword and without it) and 'chains' a call to CheckPost from this class with resulting task.

This behavior is critical, because async postconditions should be checked only when the task completes, but the logic is wrong for async postconditions in generic methods:

class Foo
{
  public static async Task<T> FooAsync() where T: class
  {
    Contract.Ensures(Contract.Result<T>() != null);
    await Task.Delay(42);
    return default(T);
  }
}

In this case synchronous postcondition is emitted:

// after rewriting
class Foo
{
  public static async Task<T> FooAsync() where T: class
  {
    // some stuff generated by C# compiler
    var task = ...

   RewriterMethods.Ensure(task.Result != null, "Contract.Result<T>() != null");
    return default(T);
  }
}

[SergeyTeplyakov]

This bug is related to the #235 and I'll push PR with both fixes in a couple of days.