The current `'a` lifetime in the `Device` trait is essentially a workaround for lack of GATs.
I'm just experimenting how this would look like, it'll have to wait until GATs are stable to go in.
The main benefit is structs implementing `Device` can now borrow stuff. This wasn't possible
before because the `for<'d> T: Device<'d>` bounds would essentially imply `T: 'static`.
The use of this type has several drawbacks:
* It does not allow distinguishing between different error
conditions. In fact, we wrongly conflated some of them
before this commit.
* It does not allow propagation via ? and requires manual use
of map_err, which is especially tiresome for downstream code.
* It prevents us from expanding the set of error conditions
even if right now we have only one.
* It prevents us from blanket using Result<T> everywhere
(a nitpick at most).
Instead, use Result<T, Error> everywhere, and differentiate error
conditions where applicable.
Various parts of smoltcp require an arrow of time; a monotonically
increasing timestamp. Most obviously this is TCP sockets, but
the tracer and the pcap writer devices also benefit from having
timestamps. There are a few ways this could be implemented:
1. using a static Cell, global for the entire smoltcp crate;
2. using a static method on Device;
3. using an instance method on Device;
4. passing the current timestamp into *Interface::poll.
The first two options are undesirable because they create a notion
of global clock, and interfere e.g. with mocking.
The third option is undesirable because not all devices are
inherently tied to a particular clock, e.g. a loopback device isn't.
Therefore, the timestamp is injected into both sockets and devices
through the *Interface::poll method.
