Introduce type-based linking of requests and replies

[sosthene-nitrokey]

This ensures that PollClient::request always matches the correct Request and Reply

This patch also fixes #84 and replaces the From that used unreachable_unchecked with TryFrom.

The main additions are the traits:

pub trait RequestData: Into<Request> + TryFrom<Request, Error = crate::Error> {
    type CorrespondingReply: ReplyData<CorrespondingRequest = Self>;
}

pub trait ReplyData: Into<Reply> + TryFrom<Reply, Error = crate::Error> {
    type CorrespondingRequest: RequestData<CorrespondingReply = Self>;
}

And PollClient's request becomes

// old
fn request<T: From<Reply>>(&mut self, req: impl Into<Request>) -> ClientResult<'_, T, Self>;
 // new
fn request<Rq: RequestData>(
      &mut self,
      req: Rq,
) -> ClientResult<'_, Rq::CorrespondingReply, Self>;

This also showed that the reply type for the RemoveDir was incorrect (it returned RemoveDirAll) which actually triggered UB is mentionned in #84

[sosthene-nitrokey]

I wonder if this checking could also be used on the backend side.

[robin-nitrokey]

What do you think about dropping the Corresponding prefix? Would be a bit easier to read.

[sosthene-nitrokey]

It gets confusing to have multiple types named Request and Reply, but I agree it is a bit verbose. I initially added it because as I was dealing with the errors that came with the refactor it was confusing.

I removed it in 79ccc04

[robin-nitrokey]

I generally agree, but for associated types I think re-using existing names is fine (e. g. TryFrom::Error).

[nickray]

This is super cool, it always bugged me that these types were not matched on a type level.