Disallow importing private items

[catamorphism]

resolve wasn't checking that a use referred to a public item.

r? @brson

[brson]

nice! r+

[brson]

I'm seeing some oddities, presumably as a side effect of this patch.

I have a mod like

pub mod flatteners {
    pub struct SerializingFlattener { ... };
    impl SerializingFlattener { ... };
}

And I can no longer import flatteners::SerializingFlattener because the impl isn't pub (changing it to pub works). Is this intentional?

[catamorphism]

It's not intentional. Are impls and structs separate namespaces?

[catamorphism]

Patch is merged -- @brson , open a bug for the struct/impl thing if necessary.