librustc: Forbid private types in public APIs.

This breaks code like:

    struct Foo {
        ...
    }

    pub fn make_foo() -> Foo {
        ...
    }

Change this code to:

    pub struct Foo {    // note `pub`
        ...
    }

    pub fn make_foo() -> Foo {
        ...
    }

The `visible_private_types` lint has been removed, since it is now an
error to attempt to expose a private type in a public API. In its place
a `#[feature(visible_private_types)]` gate has been added.

Closes #16463.

RFC #48.

[breaking-change]