Appetite for refactoring to be SANS-IO (including time?)

This library is basically SANS-IO except for calls to `Instant::now`. I was wondering what the appetite is for changing that?

- APIs like `encapsulate` and `decapsulate` should likely take an `now: Instant` parameter to accurately reflect timestamps of last received packets.
- `update_timers` should also take a `now: Instant` parameter.

`update_timers` can save the provided `now` timestamp as local state and all other functions can compute durations etc based on that timestamp. Thus, accuracy of that depends entirely on the user in how often they call `update_timers`.

We might want to consider renaming `update_timers` to something like `handle_timeout` or `update_now`.