New proc macro (#387)

* Start working on the new proc macro system

* Add skeleton for rendering

* Improve error reporting

* Main part of 'render_client'

* Implement RPC client generation

* Client successfully rendered

* Add doc-comment generation for the API client

* Check that all the methods have receiver

* Start working on the server impl

* Add helper method to find server crate

* Fix usage of client rpc path

* Decent progress on the server macro implementation

* Server macro compiled successfully

* Remove unneeded re-export

* Insert SubscriptionSink argument to the subscription server signatures

* Add basic doc-comment for the macro

* no_run -> ignore

* Trait with subscription compiles

* Extend the example

* Add integration test for client/server impl

* Add trybuild setup

* Set correct span for attribute parsing related errors

* Add basic set of trybuild tests

* Add tests for client and server generated separately

* Improve proc-macro documentation

* Update proc-macros/src/lib.rs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* Fix a couple of bugs in docs

* Fix rendering subscription with params

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>