refactor: move wrap into separate attribute, cleanup derive macro code (#55)

This has a couple of refactorings on top of #51:

* Move the `wrap` functionality into a `wrap` attribute on variants, and adjust docs accordingly. I think this is better, it is a separate feature from the rpc types, and the rpc attr easily gets too long for a single line otherwise.
* Add support to set visibility and additional derives for `wrap` types. This makes this far more useful, otherwise you can't use the wrap argument as soon as you need a derive (e.g. Clone) on the generated type. By refactoring the code a bit, this barely adds more code to the macro.
* Make `tx` argument in `rpc` attribute optional, and change parsing to not use a map but just look for the only two arguments (`rx` and `tx`)
* Refactor the derive crate to be a bit simpler, and easier to follow by putting the main macro up to the top of the file
* Adds a full example to crate docs

The refactor of the `derive` crate has no changes apart from the `wrap` changes outlined above.

Author: Frando