We should establish conventions for how to expose the implementation details of macros as parts of the public API of a crate. Currently we have two "schemes"

• Prefix the name with __ example
• Liberal use of #[doc(hidden)] example

It would be nice to standardize on the convention here as most of these internals "we'd really rather not expose". I personally like the double-underscore-prefix-plus-#[doc(hidden)] strategy but others may feel differently!

Nominating, this may tidy up some of our apis in std.