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.

Author: jyn514

library/core/primitive_docs/box_into_raw.md

@@ -0,0 +1 @@
+../std/boxed/struct.Box.html#method.into_raw

library/core/primitive_docs/fs_file.md

@@ -0,0 +1 @@
+../std/fs/struct.File.html

library/core/primitive_docs/io_bufread.md

@@ -0,0 +1 @@
+../std/io/trait.BufRead.html

library/core/primitive_docs/io_read.md

@@ -0,0 +1 @@
+../std/io/trait.Read.html

library/core/primitive_docs/io_seek.md

@@ -0,0 +1 @@
+../std/io/trait.Seek.html

library/core/primitive_docs/io_write.md

@@ -0,0 +1 @@
+../std/io/trait.Write.html

library/core/primitive_docs/net_tosocketaddrs.md

@@ -0,0 +1 @@
+../std/net/trait.ToSocketAddrs.html

library/core/primitive_docs/process_exit.md

@@ -0,0 +1 @@
+../std/process/fn.exit.html

library/core/primitive_docs/string_string.md

@@ -0,0 +1 @@
+../std/string/struct.String.html

library/core/src/bool.rs

@@ -2,7 +2,8 @@
 
 #[lang = "bool"]
 impl bool {
-    /// Returns `Some(t)` if the `bool` is [`true`](keyword.true.html), or `None` otherwise.
+    /// Returns `Some(t)` if the `bool` is [`true`](../std/keyword.true.html),
+    /// or `None` otherwise.
     ///
     /// # Examples
     ///
@@ -18,7 +19,8 @@ impl bool {
         if self { Some(t) } else { None }
     }
 
-    /// Returns `Some(f())` if the `bool` is [`true`](keyword.true.html), or `None` otherwise.
+    /// Returns `Some(f())` if the `bool` is [`true`](../std/keyword.true.html),
+    /// or `None` otherwise.
     ///
     /// # Examples
     ///