auto merge of #12017 : FlaPer87/rust/replace-mod-crate, r=alexcrichton

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.

diff --cc src/librustc/middle/typeck/check/mod.rs
Simple merge

diff --cc src/librustdoc/clean.rs
Simple merge

diff --cc src/librustdoc/html/format.rs
Simple merge

diff --cc src/libsyntax/ext/base.rs
Simple merge

diff --cc src/libsyntax/ext/expand.rs
Simple merge