... like it will be in Chalk. We still keep `interned_mut` and
`into_inner` methods that will probably not exist with Chalk.
This worsens performance slightly (5ginstr inference on RA), but doesn't
include other simplifications we can do yet.
It's very useful when `pub` is equivalent to "this is crate's public
API", let's enforce this!
Ideally, we should enforce it for local `cargo test`, and only during
CI, but that needs https://github.com/rust-lang/cargo/issues/5034.
