quinn-udp: Support vectored writes

[kixcord]

The Transmit struct accepts a single buffer. If it accepted a &[IoSlice] then I could perform vectored writes.

My use case is actually for WebRTC. I want to batch/encode RTP packets but they have a variable header size due to extensions. I can only add up to 255 bytes of padding for each RTP packet so there's a real limit to what can be batched.

Currently, I have two passes:

• Encode each header to a temporary buffer to learn the size.
• Figure out the next N packets that can be batched.
  • header.size + payload.size has to be within 255 bytes.
• Encode the next N packets.
  • Copy the header from the temporary buffer.
  • Encode the payload.
  • Encode padding to equal the segment size.
  • Encrypt.
• Send the next N packets in one buffer.

If we supported vectored writes, I think this could be simplified to:

• Encode each packet to its own buffer.
• Figure out the next N packets that can be batched.
  • buffer.size has to be within 255 bytes
• Encode the next N packets
  • Encode padding to equal the segment size.
  • Encrypt.
• Send the next N packets via vectored IO.

The headers aren't huge but it's nice to avoid a copy and additional logic to compute the size. I could alternatively add a header_size() method to compute the header size before encoding, which is what I did with my old QUIC library, but I don't like how the implementations can drift.

Also I know vectored IO works with GSO because I implemented it at my last company. I was using a segment per buffer but that's not explicitly required.