Can currently leak private types and values

[baszalmstra]

Reexporting a type or value from one module to another should check the visibility of the original item.

This can happen in a few cases:

//- foo.mun
// Illegal, Bar has a smaller scope that this use statement
pub(super) struct Bar;

// Illegal, Bar has a smaller scope than this function
pub fn baz() -> Bar {
}

//- mod.mun
pub use foo::Bar; // Should be illegal because Bar is only visible to this module

In rust this emits the error: can't leak private type. This is currently not checked in Mun.

[mkhan45]

We also need to check that a public struct does not contain any private types in its exported fields:

struct Foo;

pub struct Bar {
    pub foo: Foo // illegal since Foo has a smaller scope than Bar
}

[mkhan45]

this is closed by #338 and #344

[baszalmstra]

Nice! Thanks @mkhan45 !