Remove some Unpin requirements on Futures

[tomaka]

Opening as a draft.
The goal is to remove all Unpin requirements on futures on traits such as *Upgrade or Transport.
All the T::Dial: Unpin/T::Listener: Unpin/T::ListenerUpgrade: Unpin requirements still remain to be cleaned up.

This is surprisingly hard and my progress has stalled when chaining multiple futures is involved.
I've used unsafe code for transport::and_then, but that should ideally be avoided unless no other solution is found.

If someone wants to take over this work, let me know.

[tomaka]

This is ready for review!

I ended up cheating a bit by using Pin<Box<...>> where there was no other solution. All the unsafe code has been removed.

There's a breaking change: since the Transport::Listener no longer necessarily implements Unpin, I had to modify remove_listener to not return the Listener stream anymore.

[tomaka]

Note that I removed all the Unpin requirements on futures and streams related to Transports and upgrades, but I haven't touched the ones related to AsyncRead/AsyncWrite. There are around 180 occurrences of Unpin related to AsyncRead/AsyncWrite to remove, so that's for later.

[twittner]

The goal is to remove all Unpin requirements on futures on traits such as *Upgrade or Transport. [...] There are around 180 occurrences of Unpin related to AsyncRead/AsyncWrite to remove, so that's for later.

Do you have upgrade, transport or i/o types in mind which must not be moved and where our Unpin bounds present a problem?

[tomaka]

Do you have upgrade, transport or i/o types in mind which must not be moved and where our Unpin bounds present a problem?

Anything that uses async/await does not implement Unpin. #1383 is a concrete example.