Update docs to fix various 404s

Found a few 404s that seemed like simple fixes:

The Result docs use old_io Writer as an example. Fix the link to old_io Writer. There's probably an effort to update the example away from a deprecated api but this was a simple fix.

rustc/plugin was pointing at the old guide and it was a broken link anyways (plugin vs plugins). Point at the book instead.

The main page of the API docs referenced c_{str,vec}. Looks like these were deleted in 25d5a3a194. Point at ffi docs instead.

src/librustc/plugin/mod.rs

@@ -47,7 +47,7 @@
 //! #![plugin(myplugin)]
 //! ```
 //!
-//! See [the compiler plugin guide](../../guide-plugin.html)
+//! See the [Plugins Chapter](../../book/plugins.html) of the book
 //! for more examples.
 
 pub use self::registry::Registry;