Tracking issue for code generation

[ubnt-intrepid]

Overview

An attribute-style procedural macro is introduced in order to modify the signature of handler functions into an appropriate form to be used by the framework:

#[handler]
fn handler() -> impl Responder { ... }

Tasks

• Add tsukuyomi-codegen and implement the initial support (add custom attributes to modify the signature of handler functions #78).
• Adjust the core API (some breaking changes may be required).
• (Optional) wait for proc_macro and use_extern_macros to be stabilized.
• Release tsukuyomi-codegen

[ubnt-intrepid]

note: tsukuyomi-codegen is re-added with the new extract API.