librustc: Give trait methods accessible via fewer autoderefs priority

[pcwalton]

over inherent methods accessible via more autoderefs.

This simplifies the trait matching algorithm. It breaks code like:

impl Foo {
    fn foo(self) {
        // before this change, this will be called
    }
}

impl<'a,'b,'c> Trait for &'a &'b &'c Foo {
    fn foo(self) {
        // after this change, this will be called
    }
}

fn main() {
    let x = &(&(&Foo));
    x.foo();
}

To explicitly indicate that you wish to call the inherent method, perform
explicit dereferences. For example:

fn main() {
    let x = &(&(&Foo));
    (***x).foo();
}

Part of #17282.

[breaking-change]

r? @nikomatsakis

[pcwalton]

@nikomatsakis No, that's not an error. Inherent methods win at each level of autoderef. See the logic in search_for_method.

[pcwalton]

@bors: retry