on Connection that describes the connection state, and implement the fns
alpn and remote_id differently depending on what the state is.

The upside is that it should be easier to write code that works with both
a 0rtt connection and a fully initialized connection without having to
define a trait to abstract over both variants.

The downside is that it is a bit fancy with the types. But then for the
non 0rtt case you would never see the fanciness, so maybe it is OK?

…... structs

Note: this requires using .connection() when you want to do something with the
connection. You could avoid this with Deref, but it seems this way of using
Deref is frowned upon in rust.

they are now just type aliases for Connection<Incoming...> and
Connection<Outgoing...>

With this the usage is idential to before. No more .connection() needed
for e.g. .open_bi().

- export them and the trait
- make the data types doc(hidden)