Update Unstable Book links to reflect to new layout.

Fixes rust-lang/rust#41295.

first-edition/src/unsafe.md

@@ -139,4 +139,4 @@ I’ll repeat again: even though you _can_ do arbitrary things in unsafe blocks
 and functions doesn’t mean you should. The compiler will act as though you’re
 upholding its invariants, so be careful!
 
-[intrinsics]: ../../unstable-book/intrinsics.html
+[intrinsics]: ../../unstable-book/language-features/intrinsics.html

first-edition/src/using-rust-without-the-standard-library.md

@@ -9,7 +9,7 @@ don’t want to use the standard library via an attribute: `#![no_std]`.
 > Note: This feature is technically stable, but there are some caveats. For
 > one, you can build a `#![no_std]` _library_ on stable, but not a _binary_.
 > For details on binaries without the standard library, see [the nightly
-> chapter on 'lang items'](../../unstable-book/lang-items.html#using-libc)
+> chapter on 'lang items'][unstable book lang items]
 
 To use `#![no_std]`, add it to your crate root:
 
@@ -40,3 +40,5 @@ fn may_fail(failure: bool) -> Result<(), &'static str> {
     }
 }
 ```
+
+[unstable book lang items]: ../../unstable-book/language-features/lang-items.html#using-libc

redirects/compiler-plugins.md

@@ -1,4 +1,6 @@
 % The Rust Programming Language Has Moved
 
 This chapter of the book has moved to [a chapter in the Unstable
-Book](../unstable-book/plugin.html). Please check it out there.
+Book][unstable book plugins]. Please check it out there.
+
+[unstable book plugins]: ../unstable-book/language-features/plugin.html