Help stabilize a subset of Macros 2.0!

[RalfJung]

@alexcrichton

Dealing with editions doesn’t come into hygiene, these macros will basically allow inserting names into a module which other items can also reach, just like macro_rules! does today. Many questions can basically be answered “what does macro_rules! do today?”

So what happens if a new edition introduces a new keyword, and hence parsing depends on the edition. Will the macro-generated code parsed with the edition of the macro, or the file where the macro is used?