Allow importing from the current scope

[wthrowe]

One of the big arguments in favor of RFC #390 (enum namespacing) was that it was easy to get the old behavior back using an import. There is one case, however, where this is not possible: an enum defined in an unnameable scope. I think the only place in current Rust where one can do this is inside a function.

mod a {
    fn b() {
        enum Foo { Bar }
        use Foo::Bar; // Tries to find to Foo in the crate scope (or as an external crate).
        use self::Foo::Bar; // Tries to find a::Foo.
    }
}

I haven't come up with a good syntax to propose for this operation, though.

[wthrowe]

And for completely unrelated reasons I just happened to stumble upon the related rust-lang/rust#23314.

[wthrowe]

Duplicate of #959.