Implement DerefAsync and DerefMutAsync as Deref and DerefMut in async context

[ijsgaus]

Introduce traits:

pub trait DerefAsync {
    type Target: Deref<Target::T>;
    async fn derefAsync(&self) -> Async<T>;
}

impl Deref for T: DerefAsync<Target::TOut> + TOut: Deref<Target :: TOut1>
{
   type Target : Async<TOut1>
   fn deref(&self) {
      self.derefAsync().AndAlso(move |t| t.deref());
   }
}

Add support for awaiting derefs. This can be very useful, EG, in async realization of lazy pattern:

   let y = 5;
   let lazy = async_lazy!(y, async |a| await!(a + 2));  
   let x : i32 = await!(lazy);

[ishitatsuyuki]

Can you comment on the await RFC PR or once it merged, the tracking issue? Language decisions should always happen there.

[ijsgaus]

@ishitatsuyuki Can you give me reference link, i don't know, where it is

[sfackler]

rust-lang/rfcs#2394