"Static" methods should not be object safe

[nikomatsakis]

While reading the object safety code I found this:

ty::StaticExplicitSelfCategory => {
    // Static methods are always object-safe since they
    // can't be called through a trait object
    return msgs
}

I think this is sadly incorrect. The whole idea of object safety is to ensure that an object type Foo implements the trait Foo. But if that is the case, then we can have an example like this one:

trait Foo for Sized? {
    fn make() -> Self; // static method
}

fn use<Sized? F:Foo>() {
    let x = Foo::make();
}

fn main() {
    use::<Foo>();
}

cc @nick29581
cc @aturon

[nikomatsakis]

I hope that fixing this doesn't cause another round of annoyed users. :(

[ftxqxd]

See #18527, the PR that introduced this, and rust-lang/rfcs#428, which is the same issue as this one.