refactor: Introduce a TransmitBuf for poll_transmit

[flub]

Hi, I've been looking around in poll_receive recently and started wondering if it could be simplified. This is my first attempt at a step in this direction, it pulls together the state about datagrams and their boundaries in the write buffer. I think this helps understand the interactions between a bunch of state changes that were previously spread across many lines. See the commit message for a detailed description.

If the general direction is considered useful I think I'd like to continue this further. The next thing I'd probably look at is the buffer management for a packet. I expect a lot of places don't need to know much about the full transmit buffer they're writing into but rather need to know the (start, len, end) space they can write their frames. Probably reducing the need to do everything with offsets from the transmit buffer.

Anyway, my aim is the make things clearer and easier to manage. Let's first see if this is step in that direction and then see what the future steps turn out to be. I think any change like this should be able to stand on its own rather than rely on a future change, because the future is always uncertain and might never happen :)