Test proc macros

We cannot check proc macro entirely for now but there are a few things we could check:

• proc_macro and derive declarations are not used within a regular context
• no error message is emitted when a declaration occur within a proc macro
• eventually add a scan assembler test for the proc macro output, we could make sure the proper symbols are emitted
• functions tagged with #[proc_macro] must be `pub
• function-like proc macro has incorrect signature

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro]
pub fn my_proc_macro(ts: TokenStream) -> TokenStream {
    ts
}