rust private

Document the `proc_macro` feature in the Unstable Book Discusses the `proc_macro` feature flag and the features it enables: * Implicit enable of `extern_use_macros` feature and how to import proc macros * Error handling in proc macros (using panic messages) * Function-like proc macros using `#[proc_macro]` and a usage example for creating and invoking * Attribute-like proc macros using `#[proc_macro_attribute]` and a usage example for creating and invoking [Rendered](https://github.com/abonander/rust/blob/book_proc_macro/src/doc/unstable-book/src/language-features/proc-macro.md)