rewrite tait's -> async fn | mem::replace-> take()

[Vollbrecht]

implements #49

[MabezDev]

It's clear to me that TAIT will be stabilized before AFIT, and this seems even more apparent given that they are splitting the feature and pushing the stabilization of impl Trait in assoc types (our usecase). With embedded-io moving to AFIT quite early, it puts us in a bit of a tricky spot.

I guess to support both we need to make embedded-io 0.3 and 0.4 optional, which either can be opted in to. The problem is that using embedded-io we're locked into their choices, maybe we should petition in upstream embedded-io to switch back to TAIT (downside: more ecosystem churn).

Not sure what the best solution is here.

[Vollbrecht]

next embedded-hal wg meeting is not announced yet? adamgreig mention he is away till 2nd May

[ivmarkov]

I've implemented a different approach for now, which is heavier weight on embedded-svc but minimizes any other changes elsewhere - basically, rather than pub use-ing the async traits of embedded-io from within embedded-svc::io, I've moved them into embedded-svc::io, yet I've raised the dependency on embedded-io 0.4, as the rest of that crate can be re-used unchanged (the blocking and the common parts). There's also EmbIo adapter if somebody really really needs to pass an embedded-io-4 async trait impl where we have embedded_svc::io::asynch impl or the other way around. Not that I expect such a use-case to happen, but you never know.

[ivmarkov]

We can keep this PR open and push it (potentially - with changes) once there is extra clarity how the "real" async traits are going to look like (not the TAIT + GAT manual desugaring we are continuing to use for now). I'm however not holding my breath that async traits are going to land any time soon.

[Vollbrecht]

meta info: https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizing-async-fn-in-trait.html

[ivmarkov]

Unfortunately TAIT was not stabilized, and in the meantime AFIT is not incomplete.
The rest of the embedded community is heavily leaning on AFIT, as we are switching too - here: #56