Refactor quic traits `Connection` and `OpenStreams` to use `self: Pin<&mut Self>` instead of `&mut self`

The current quic traits have &mut self in the function signature. For example:

    fn poll_accept_recv(
        &mut self,
        cx: &mut task::Context<'_>,
    ) -> Poll<Result<Self::RecvStream, ConnectionErrorIncoming>>;

In the original proposal some of the trait methods had self: Pin<&mut Self> instead.
https://github.com/hyperium/h3/blob/master/docs/PROPOSAL.md#connections

It would take a large refactor to chance it now, but i think it is possible for the Connection and OpenStreams trait.

This would allow quic implementations like h3-quinn to not require Boxing the streams.
https://github.com/hyperium/h3/blob/bf078de3c769d2ea7b01bf6c4829c148b9ab9935/h3-quinn/src/lib.rs#L45-L48

What do you think, is it worth the time?

With a little digging and found this comment.
https://github.com/hyperium/h3/pull/3#discussion_r464525968

This comment refes to the RecvStream trait.
The problem mentioned in the comment can be worked around for Connection and OpenStreams which only have two methods not needing Pin , which is close and opener.
Either by defining a new separate trait for close and opener or by maintaining a separate cloneable instance of OpenStreams which is not pinned.

Start with the Connection and OpenStreams trait definitions and compare their signatures with the connections section of docs/PROPOSAL.md. Read h3-quinn/src/lib.rs around the referenced stream implementation and the discussion in PR #3 to understand the pinning constraint. Done means agreeing on the trait design and completing the resulting refactor without requiring boxed streams in h3-quinn.