Add primitive documentation to libcore

This works by doing two things:
- Adding links that are specific to the crate. Since not all primitive
  items are defined in `core` (due to lang_items), these need to use
  relative links and not intra-doc links.
- Duplicating `primitive_docs` in both core and std. This allows not needing CARGO_PKG_NAME to build the standard library. It also adds a tidy check to make sure they stay the same.

library/core/src/lib.rs

@@ -130,6 +130,7 @@
 #![feature(decl_macro)]
 #![feature(doc_cfg)]
 #![feature(doc_notable_trait)]
+#![feature(doc_primitive)]
 #![feature(exhaustive_patterns)]
 #![feature(extern_types)]
 #![feature(fundamental)]
@@ -355,3 +356,5 @@ pub mod arch {
         /* compiler built-in */
     }
 }
+
+include!("primitive_docs.rs");