This also seizes the opportunity to move integer literal parsing to
the syntax crate, were it logically belongs.
Note though that this is still done in an ad hoc manner -- we probably
should split kitchen sink ast::Literal into a separate APIs for
strings, ints, etc
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.
