Add a crate keyword and allow it to link external crates

The first setp for #9880 is to add a new `crate` keyword. This PR does exactly that. I took a chance to refactor `parse_item_foreign_mod` and I broke it down into 2 separate methods to isolate each feature.

The next step will be to push a new stage0 snapshot and then get rid of all `extern mod` around the code.