async: User async-function-in-trait

[chrysn]

This is a WIP attempt to use the recent async-function-in-trait feature. It makes embedded-nal-async regress into nightly-only, but then again the feature(generic_associated_types) was not even removed yet, and it's barely possible to actually implement the traits described here without using impl Trait in the associated types' definitions anyway.

Benefits

• Clearer interfaces
• Better ergonomics for trait implementers
• The provided version of connect() (that falls back to connect_from an unspecified address) is now actually provided

Downsides

• The lifetimes of the associated types of the stack (Connected etc) had to go away due to known issues. But I kind of doubt we'd really have needed them. This probably needs further discussion, but before that I'll have to understand again myself why I had them there in the first place.
• async-function-in-trait is an unstable feature (but it sounds like it would move toward stabilization fast, and it's my impression that it might be there even faster than impl Trait for associated types)

Status

This is experimentation at the moment, and only done for UDP (thus based on #73 until that is merged; only consider the top few commits for this PR). There is a branch on std-embedded-nal that already followed the switch. [edit: And because GitHub and GitLab don't federate PR mentions like modern issue trackers do: it's on https://gitlab.com/chrysn/std-embedded-nal/-/tree/async-fn-in-trait]

Porting DNS and TCP is a step on this PR's path from being a draft PR to a regular one.

[chrysn]

Closing this because it's all being included in #73.

The "it's nightly again" and "TCP/DNS is missing" are moot since #78.

I've looked at the Connected (etc) lifetimes, and think that the old lifetimes were just a needless artifact -- so I see no open discussion points worth carrying over to #73.