Implement a lint for the use of private types in exported type signatures

These are types that are in exported type signatures, but are not
exported themselves, e.g.

    struct Foo { ... }

    pub fn bar() -> Foo { ... }

will warn about the Foo.

Such types are not listed in documentation, and cannot be named outside
the crate in which they are declared, which is very user-unfriendly.

cc #10573.