move generic Buffer parameter to fn #94
                
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Solves #93
The idea is that the quic abstraction does not need the send_data method with its protections. h3::quic::SendStream only has poll_write() and we use that in the higher levels to implement send_data() as an async fn.
The user can not accidentally have two running send_data() calls, because the async fn takes a mutable borrow of the RequestStream.