Add support for handling re-exports and visibility

[udoprog]

A re-export is when an item is imported into a file, like so:

mod a {
    pub use std::option::Option as Bar;
}

fn main() {
    a::Bar::None is Option
}

This requires a bit of care, since the import system is orthogonal to the compile meta system. The hope is that they can be merged at some point.

Note: access checking is still only checked once, from where something is used to where it's declared. So we currently don't support access to get around visibility (yet). Uses also aren't cached, but this will happen once the import system is moved into Query, instead of UnitBuilder where it is now.

We do detect cycles at this point. And support wildcards:

mod a {
    pub mod c { pub use std::option::Option as Baz; }
    pub mod b { pub use super::c::Baz as Bar; }
    pub use self::b::{Bar as Foo, Bar as Foo2};
}

use self::a::*;

fn main() {
    (Foo::None is Option, Foo2::Some(2) is Option)
}