using OutBodyIface

[kazu-yamamoto]

This catches up the changes in kazu-yamamoto/http-semantics#2

@edsko I think I should fix the code marked with fixme. QUIC's FIN can send by shutdownStream. But I don't know how to interact with the trailer maker.

[edsko]

I think the trailers maker actually is fine. We could conceivable tell it that this chunk is the final one, but it's not critical (at least, it's not for me). This PR looks fine me, except that perhaps pushFinal does not actually provide any guarantees over push; but perhaps that is not necessary for HTTP3? Maybe the only thing that needs to change is that the documentation over outBodyPushFinal needs to change a bit, and should just say something like "this information may be used to ensure that..", rather than the stronger wording it currently has?

[kazu-yamamoto]

Merged. Thanks!