Better names for h3::quic

The name poll_accept_recv is a bit counter-intuitive. Is it accepting the peer's request for receiving? But it's actually meant for accepting a unidirectional (send) stream from the peer, or in the caller's perspective, opening a stream for receiving. So it may not be a good name for the case, as it requires people to do a bit interpretation to figure out what the function is supposed to do.

quinn's API Connection::accept_uni feels much more intuitive, accepting unidirectional communication from the peer. And it's clear that it's for receiving in the context.

Proposal for API changes:

• poll_accept_recv() -> poll_accept_uni()
• poll_open_send() -> poll_open_uni() (to be symmetric and predictable)
• Maybe?
  • poll_accept_bidi() -> poll_accept_bi() (as poll_accept_bi is more predictable when we have poll_accept_uni)
  • poll_open_bidi() -> poll_open_bi() (to be symmetric)

Locate the public API occurrences of poll_accept_recv, poll_open_send, poll_accept_bidi, and poll_open_bidi, then review the naming proposal against quinn's Connection::accept_uni API. Done means the agreed names are applied consistently and the affected tests or documentation use the updated API.