remove Unpin requirement for the inner stream

[BiagioFesta]

Currently, WebSocketStream requires Unpin being implemented for the underlying I/O resource.

The aim of this PR is to remove this constrain.

Context

Any underlying I/O streams that work with timeouts logic won't implement Unpin.

For example: tokio-io-timeout::TimeoutStream it does not implement Unpin.

That's because any timer related future over tokio runtime is !Unpin.

Of course, an easy solution would be to allocate the underlying stream on the heap for pinning (e.g., Pin<Box<T>>). However, that means heap allocation and pointer indirection. Although those drawbacks are probably not really impactful, removing Unpin requirement will make this library more flexible.

On the other hand, removing Unpin does not come for free:

1. Adding additional dependency: pin_project.
   • I think this is not a real concern. pin_project is very very widely spread. It is likely indirectly pulled in anyway.
2. unsafe code.
   • This is real drawback of this PR. The tread-off here is: library more flexible but with unsafe blocks.

[BiagioFesta]

missing SAFETY comment:

// SAFETY: not moving out

[Dushistov]

think this is not a real concern. pin_project is very very widely spread

tokio depend on pin-project-lite, not pin-project. futures-util also depends on pin-project-lite.

[BiagioFesta]

think this is not a real concern. pin_project is very very widely spread

tokio depend on pin-project-lite, not pin-project. futures-util also depends on pin-project-lite.

I guess we can switch to pin-project-lite, still not the real concern of this change I guess