More possibilities with Modbus-Exceptions

[emcell]

This got a little bigger than I initially expected. I hope you're ok with my changes.

What I've done so far

• Made some types of frame module public
• Converted FunctionCode to an enum and replaced it in a lot of places in codec module. I've done this because I needed the function code in the service handlers to send the Correct ExceptionResponse object.
• Added the GetFunctionCode trait and implemented it on Request and Response. This is also necessary to generate the correct ExceptionResponse in the handlers.
• Added the ExtractExceptionResponse trait. This is for an easy way to downcast the std::io::Error in the clients to ExceptionResponse
• Added a new example that combines the following things:
  • async tcp-server
  • async rtu-server
  • An example of a Service that really uses async and not futures::ready
  • Specifying ExceptionResponses in the Service Layer
  • extracting ExceptionResponse from the client response

I started this because I didn't find ways to specify Modbus-Exception and didn't find a way to extract those in the clients.

I Think this PR should fix #169 and #98.

What could be better here?

I think It would be better to be able to Just return a Result<Response, Exception> from the Service and figure the correct function_code out after the service handler and automatically convert the Exception into ExceptionResult. But I didn't want to make an even bigger change without speaking to to you in the first place.

[uklotzde]

Thanks for contributing!

IMHO this PR contains too many unrelated changes. The FunctionCode newtype should be introduced by a separate PR.

[emcell]

any chance this is getting merged soon?

[uklotzde]

Please give this PR a concise title that later becomes the Git commit message.

We also need a changelog entry that summarizes the changes and consequences for users of this crate. Ideally a single sentence for every notable API change. As short as possible.

Everyone is invited to do a review. We are managing this crate in our spare time without any monetary compensation. Please understand that paid projects might be prioritized. Thank you.

[uklotzde]

Should be rebased and updated once #246 has been merged.

[uklotzde]

I will close this PR after seeing no progress lately and because merge conflicts are pending.

Please open a new PR whenever you have something ready. Thank you.