move generic Buffer parameter to fn

[yu-re-ka]

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.

[yu-re-ka]

I still think this is the right way to do it, making both the code and the api cleaner. But please let me know your opinion so that I can either close or rebase this.