Implement `cfg(accessible)` using `ParseCallbacks::int_macro`

[workingjubilee]

suggested by @usamoi in #2162 (comment)

I decided to defer this to a future-work improvement because it should be noncritical for practical usage of the struct. Programmers should resist the temptation to allocate random Postgres structs or prod random fields. They certainly shouldn't be doing that for left-private-on-purpose fields.