Refine server proc macros

[dvdplm]

• remove old macros
• test the new ones in substrate
• support generic type params in in trait definition [new proc macros]: doesn't support generic types in API definition. #426
• Add more UI tests.
• Add tests for missing parameter deserialization (if the parameter is an Option, it should still work).
• Improve auto-generated doc-comments:
  • Subscriptions descriptions should include the type of yielded items.
  • Provide a way for users to extend auto-generated doc-comments (there must be a way to add descriptions to both server- and client-side parts).
• Rewrite no_run examples in doc-comments to the actual doc-tests (otherwise they'll become outdated at some point).
• Add a way to set up an associated type for the RPC-specific Error type or just return the actual error instead of putting it inside Ok(Err(err)) which causes the error type to have the Serialize bound, Proc macro tweaks #403 (comment)
• Require subscriptions to return an error, just like for normal methods

(do add more details here as needed, draft ticket)