Allow in-place unwrapping of future::Ready

[tyranron]

This PR adds into_inner method for future::Ready.

Motivation

Due to lack of supporting async fn or impl Trait in traits at the moment, there are a lot of situations we are made to introduce redundant boxing just for "things to work".

impl<T> SomeTrait for AnotherType<T> 
where 
    T: SomeTrait<Future = future::Ready<u8>> ,
{
    type Future = LocalBoxFuture<'static, u32>;

    fn do_some(&self) -> Self::Future {
        self.0.do_some().map(|v| u32::from(v) + 32).boxed_local()
    }
}

But if we know the concrete Future's type being future::Ready, we could have been able to avoid this redundant boxing if we have direct access to the inner immediately ready value:

impl<T> SomeTrait for AnotherType<T> 
where 
    T: SomeTrait<Future = future::Ready<u8>> ,
{
    type Future = future::Ready<u32>;

    fn do_some(&self) -> Self::Future {
        future::ready(self.0.do_some().into_inner().into() + 32)
    }
}

[cramertj]

Looks reasonable to me!