Detect stability attributes on modules and crates

[huonw]

// some_crate.rs
#[experimental];

#[deprecated]
pub mod some_mod {
    pub fn foo() {}
} 

extern mod some_crate; // warning: use of experimental item

use some_crate::some_mod; // warning: use of deprecated item

fn main() {
    some_crate::some_mod::foo(); // warning: use of deprecated item
}

It seems unlikely that this should flag crates and modules without annotations, but I'm not really sure.

(Part of #6875, continuation of #8921.)

[brendanzab]

It would be great if this could be integrated with rustdoc. Perhaps allow for adding an explanatory message?

Edit: @huonw directed me to #8965

[flaper87]

I'll work on this one